Luteal Phase Temperature Rise: What to Expect and How to Track the Trend

A temperature rise after ovulation is one of the most recognizable physiological patterns of the menstrual cycle.

During the luteal phase, progesterone commonly shifts thermoregulation toward a slightly higher temperature. That change may appear in basal body temperature and can also be visible as a higher nighttime skin-temperature trend.

However, the pattern is rarely as simple as one low reading followed by one high reading.

Real menstrual cycles vary. The temperature rise may develop gradually, fluctuate from night to night, remain elevated for different lengths of time, and fall before or around the start of the next period.

Most importantly, one temperature point cannot confirm ovulation, determine the exact day ovulation occurred, or establish that pregnancy risk has ended.

The more useful approach is:

Baseline → Sustained Shift → Luteal Pattern → Period Reset → Multi-Cycle Comparison

This guide explains what a luteal-phase temperature rise usually looks like, how long it can last, why individual patterns differ, and how to interpret finger skin-temperature trends without confusing them with core body temperature or traditional basal body temperature.

Quick Answer: What Happens to Temperature During the Luteal Phase?

After ovulation, the follicle that released the egg becomes the corpus luteum.

The corpus luteum produces progesterone.

Progesterone has a thermogenic effect, so resting body temperature commonly shifts upward during the post-ovulation luteal phase.

A typical sequence is:

Follicular Baseline → Ovulation → Progesterone Rise → Higher Luteal Temperature → Progesterone Falls → Period Begins

For many people:

• Temperature is relatively lower during the follicular phase.
• A sustained upward shift appears after ovulation.
• Temperature remains relatively higher during much of the luteal phase.
• Night-to-night fluctuations still occur.
• Temperature generally moves downward near the next period when pregnancy has not occurred.

The exact size, timing, and duration of the change vary from person to person and from cycle to cycle.

What Is the Luteal Phase?

The luteal phase is the portion of the menstrual cycle after ovulation and before the next menstrual period.

After an egg is released, the remaining ovarian follicle forms the corpus luteum.

The corpus luteum produces progesterone along with estrogen.

Progesterone supports changes in the uterine lining and also influences thermoregulation.

If pregnancy does not occur, the corpus luteum eventually regresses, progesterone levels decline, the uterine lining is shed, and a new menstrual cycle begins.

How Long Does the Luteal Phase Last?

The commonly quoted average is approximately 12–14 days.

Clinical references also recognize meaningful individual variation, with approximately 10–17 days possible in healthy cycles.

The most important point is that the luteal phase is not automatically:

Day 15 through Day 28

in every menstrual cycle.

If ovulation occurs later, the luteal phase starts later.

If ovulation occurs earlier, it starts earlier.

Your Luteal Phase Is Not Necessarily Exactly 14 Days Every Month

The luteal phase is often less variable than the follicular phase, but it still changes.

Prospective research following women across many menstrual cycles has shown measurable within-person variation in luteal-phase length.

This means calendar rules such as:

“My next period is always exactly 14 days after ovulation”

can provide a rough reference but should not be treated as a biological guarantee.

Why Does Temperature Rise After Ovulation?

The post-ovulation temperature shift is primarily associated with progesterone.

Progesterone influences thermoregulatory centers in the brain and shifts temperature regulation toward a slightly higher level.

Research on core body temperature generally finds that the luteal phase is approximately:

0.3–0.7°C warmer

than the pre-ovulation follicular phase on average.

The difference is often particularly visible during sleep or immediately after waking, before activity adds additional thermal variation.

Does Temperature Rise Exactly When Ovulation Happens?

The relationship is close, but temperature is generally more useful as a post-ovulation signal.

Progesterone rises after ovulation.

The resulting temperature change can therefore become visible after the ovulatory event has already occurred.

This makes temperature tracking stronger for answering:

“Does my recent pattern look consistent with entering the luteal phase?”

than:

“Exactly what hour will I ovulate tomorrow?”

Temperature Tracking Is Primarily Retrospective

This distinction matters greatly for fertility interpretation.

A sustained temperature shift can support the interpretation that ovulation may already have occurred.

The most fertile days generally begin before that sustained rise becomes obvious.

Therefore, waiting for a temperature increase before identifying fertility can be too late for predicting the beginning of the fertile window.

How Much Should Temperature Rise After Ovulation?

There is no single required increase.

Different sources report different average shifts because they measure temperature using different:

• Body locations
• Devices
• Times of day
• Study populations
• Cycle-confirmation methods

Traditional BBT references commonly describe shifts beginning around approximately 0.2°C and sometimes reaching around 0.3–0.6°C or more.

Core-temperature research commonly finds an average luteal increase of approximately 0.3–0.7°C.

These values cannot be transferred directly to finger skin temperature.

Finger Skin Temperature Has Its Own Pattern

Research using continuous nighttime finger-temperature measurements has also identified higher temperatures during the luteal phase.

In one small real-world study, average nocturnal finger skin temperature differed by approximately:

0.30°C

between the follicular and luteal phases.

That number describes a study average.

It is not a minimum temperature shift that every person should see.

There Is No Ring Temperature Threshold That Confirms Ovulation

A change of:

+0.1°C

does not automatically mean ovulation did not occur.

A change of:

+0.5°C

does not automatically prove that it did.

The pattern becomes more informative when the increase:

• Persists across several nights
• Occurs at a plausible cycle phase
• Differs from the preceding baseline
• Repeats across multiple cycles

Skin Temperature Is Different From Core Body Temperature

This distinction is essential when interpreting RingConn data.

Core body temperature describes the tightly regulated internal temperature of the body.

Skin temperature reflects temperature at the body's surface.

Finger skin temperature is especially influenced by peripheral circulation and the surrounding environment.

Factors that can affect it include:

• Room temperature
• Bedding
• Sleep clothing
• Peripheral blood flow
• Alcohol
• Exercise
• Stress
• Illness
• Travel
• Ring fit

RingConn measures finger skin temperature trends. It does not measure core body temperature.

See our guide to skin-temperature tracking with smart rings for a fuller explanation.

Skin Temperature Is Also Different From BBT

Basal body temperature, or BBT, is a standardized resting-temperature method.

Traditional BBT tracking usually involves measuring temperature:

• Immediately after waking
• Before getting out of bed
• Before eating or drinking
• At approximately the same time each day

Nighttime finger skin-temperature tracking takes a different approach.

Instead of one morning measurement, a wearable can collect repeated data during sleep.

Measurement	What It Represents	Main Limitation
Core temperature	Internal temperature	Requires appropriate clinical or physiological measurement
BBT	Resting morning temperature	Highly dependent on measurement routine
Finger skin temperature	Peripheral nighttime temperature trend	More influenced by circulation and environment

These three measurements can show related menstrual patterns without being interchangeable.

What Does a Typical Luteal Temperature Curve Look Like?

The simplified textbook pattern is biphasic:

Lower Follicular Temperatures → Higher Luteal Temperatures

A real cycle might look more like:

Stable → Slight Dip → Gradual Rise → Variable Higher Nights → Plateau → Decline

Both can be compatible with an ovulatory cycle pattern.

Real Temperature Curves Are Often More Gradual Than Textbook Charts

Continuous wearable research suggests menstrual skin-temperature variation may be better represented as an oscillating physiological rhythm than as a perfect square-wave transition.

That means you may see:

• A gradual increase over several nights
• Small drops during the overall rise
• A mid-luteal peak
• A late-luteal decline
• A pattern that looks different from the previous month

A chart does not need to contain one obvious dramatic jump to be useful.

Why a Sustained Shift Matters More Than One Temperature Point

Consider these two examples.

Pattern A: One High Night

Night	Temperature vs Baseline
1	+0.02°C
2	+0.04°C
3	+0.42°C
4	+0.01°C
5	+0.03°C

That isolated increase has many possible explanations.

Pattern B: Sustained Higher Phase

Night	Temperature vs Baseline
1	+0.12°C
2	+0.21°C
3	+0.26°C
4	+0.23°C
5	+0.29°C

The second pattern provides stronger evidence of a phase-level shift.

Even then, temperature alone cannot clinically confirm ovulation.

Use Baseline → Shift → Persistence

A simple way to interpret the curve is:

Baseline

What did your temperature look like during the earlier portion of the cycle?

Shift

Did values begin moving above that range?

Persistence

Did the higher pattern continue across multiple nights?

Persistence provides much more information than the highest single point.

How Long Should Temperature Stay Elevated?

The higher-temperature phase generally lasts through much of the luteal phase.

Because luteal phase length varies, the high-temperature period does too.

A broad pattern might be:

Cycle Event	Temperature Pattern
Before ovulation	Relatively lower
Early post-ovulation	Upward shift begins
Mid-luteal	Higher pattern often established
Late luteal	May remain high or begin declining
Menstruation	Typically returns toward the lower cycle pattern when pregnancy has not occurred

Temperature Does Not Always Drop the Day Before Your Period

The timing of the decline varies.

Some people notice a drop:

• One or two days before bleeding
• On the first day of menstruation
• Gradually across several nights

Do not require one specific premenstrual drop pattern.

Why Your Luteal Temperature Pattern Can Change Between Cycles

Menstrual physiology is not perfectly identical every month.

Differences can reflect:

• Ovulation timing
• Progesterone patterns
• Luteal-phase length
• Sleep
• Stress
• Exercise
• Illness
• Travel
• Alcohol
• Bedroom temperature

A weaker or noisier temperature pattern in one month does not automatically mean something is medically wrong.

The Follicular Phase Usually Creates More Calendar Variability

The day of ovulation can move substantially from one cycle to another.

This changes when the luteal phase begins.

For example:

Cycle	Possible Ovulation Timing	Temperature Rise
Cycle A	Earlier	Earlier
Cycle B	Later	Later

A later temperature rise does not necessarily mean the luteal phase itself became unusually long.

It may simply reflect later ovulation.

Why Temperature Cannot Predict the Exact Day of Ovulation

The major temperature rise is associated with progesterone after ovulation.

That makes it inherently limited as a prospective marker.

When the sustained rise becomes clear, the most fertile days may already have occurred.

This is why temperature tracking is often described as more useful for retrospective confirmation of a phase shift than precise prediction of the upcoming ovulation time.

Can One Temperature Reading Confirm Ovulation?

No.

A single higher reading can be caused by:

• A warmer sleeping environment
• Alcohol
• Illness
• Exercise
• Stress
• Travel
• Measurement variation

Even several elevated readings should be treated as evidence of a pattern rather than clinical proof that an egg was released.

What Can Confirm Ovulation More Directly?

Depending on the purpose, reproductive assessment may use information such as:

• Urinary luteinizing hormone testing
• Progesterone assessment
• Ultrasound
• Other clinical evaluation

The appropriate method depends on whether the goal is general cycle awareness, conception planning, or medical fertility evaluation.

Why Temperature Alone Should Not Be Used as Contraception

This is especially important.

The fertile window begins before ovulation.

A temperature rise generally becomes apparent after the reproductive hormone changes associated with ovulation have already occurred.

Therefore, a person cannot safely assume:

“My temperature is low today, so pregnancy is impossible.”

or:

“I saw one higher temperature, so the fertile window is definitely over.”

If avoiding pregnancy is important, use an established contraceptive method or an appropriately taught and validated fertility-awareness method rather than relying on one temperature signal.

A Temperature Trend Can Support Cycle Awareness Without Being Birth Control

This distinction allows temperature tracking to remain useful without overpromising what it can do.

Temperature can help you understand:

• Whether your cycle appears to have entered a warmer post-ovulation phase
• How long your higher-temperature phase usually lasts
• Whether the pattern repeats across cycles
• When temperature usually moves downward around menstruation

It cannot guarantee pregnancy prevention.

How to Track a Luteal Temperature Trend

A useful tracking routine has six steps.

1. Record Period Day One Correctly

The first day of menstrual flow is cycle day one.

Accurate period dates provide the calendar context for interpreting later physiological patterns.

See our guide to tracking your period correctly.

2. Collect Temperature Consistently

For wearable tracking, consistent nighttime wear improves continuity.

A missing night around the expected transition can make the curve harder to interpret.

3. Establish the Earlier-Cycle Baseline

Look at your relatively lower-temperature follicular pattern.

4. Look for a Multi-Night Shift

Focus on persistence rather than one peak.

5. Follow the Elevated Phase

Observe how long the trend remains above the earlier-cycle pattern.

6. Watch the Reset Around Menstruation

Temperature commonly moves lower as progesterone declines when pregnancy has not occurred.

Track the Whole Curve, Not the Highest Number

The most useful cycle information comes from shape and timing.

A good review asks:

• When did the rise begin?
• Was it sustained?
• How long did the higher phase last?
• When did temperature begin falling?
• Did a similar pattern happen last month?

Our guide to menstrual-cycle temperature trends explains how to build this pattern over multiple cycles.

Why Several Cycles Are More Informative Than One

One cycle can contain unusual events.

For example:

• Travel
• Illness
• A major stressor
• Unusually hard exercise
• Alcohol
• Poor sleep

When a similar temperature rhythm appears across three or more cycles, your personal pattern becomes easier to recognize.

Multi-Cycle Comparison Example

	Cycle 1	Cycle 2	Cycle 3
Rise appears	Day 16	Day 18	Day 15
Higher phase	12 days	13 days	12 days
Temperature falls	Near period	Near period	Near period

The cycle day of the rise changed.

The broader post-ovulation pattern remained relatively consistent.

This illustrates why a fixed calendar day is often less useful than phase-level physiology.

Environment Can Blur a Luteal Temperature Pattern

Finger skin temperature responds strongly to the sleeping environment.

A cycle-tracking month can become noisier if you change:

• Bedroom temperature
• Heating or air conditioning
• Bedding
• Sleep clothing

Long-term tracking works best when major environmental conditions are reasonably consistent.

Exercise Can Create Temporary Temperature Noise

Hard or late exercise can influence nighttime:

• Skin temperature
• Heart rate
• HRV
• Sleep

If one luteal night becomes unusually warm immediately after a demanding workout, interpret that point in context rather than assuming progesterone suddenly increased.

Alcohol Can Change the Curve

Alcohol can influence peripheral circulation, heart rate, HRV, sleep architecture, and temperature.

A single high-temperature night after alcohol provides weaker cycle evidence than several elevated nights under your normal routine.

Illness Can Mimic a Luteal Temperature Rise

An infection or other illness can alter both core and skin-temperature patterns.

If temperature changes coincide with:

• Feverishness
• Chills
• Body aches
• Cough
• Other illness symptoms

use an appropriate clinical thermometer when assessing fever.

Finger skin temperature should not be used as a clinical fever measurement.

Stress Can Affect Peripheral Temperature

Finger temperature depends heavily on blood flow.

Stress can influence sympathetic vascular regulation and peripheral circulation.

This creates another source of short-term temperature variation.

A multi-night hormonal pattern is more useful than interpreting a brief stress-related change as a cycle event.

Travel Can Make One Cycle Look Unusual

Travel may change:

• Time zone
• Sleep schedule
• Bedroom temperature
• Alcohol consumption
• Activity
• Stress

Record travel context when comparing temperature curves across cycles.

Hormonal Contraception Can Change the Expected Pattern

Hormonal contraceptive methods alter endogenous reproductive hormone patterns in different ways.

Some methods suppress ovulation more consistently than others.

Bleeding can also represent withdrawal bleeding or breakthrough bleeding rather than menstruation following a natural ovulatory cycle.

A textbook follicular-to-luteal temperature pattern may therefore be less applicable.

Irregular Cycles Need More Context

Cycle timing can become more variable during situations such as:

• Perimenopause
• Postpartum recovery
• Breastfeeding
• Recent hormonal-contraception changes
• Other causes of irregular cycles

A wearable can still document temperature trends, but a less predictable curve should not be used to self-diagnose whether ovulation occurred.

A Missing Temperature Rise Does Not Prove Anovulation

A flat-looking wearable curve can result from:

• Measurement gaps
• Environmental noise
• Smaller individual temperature changes
• Irregular timing
• Illness
• Hormonal factors

Temperature data alone cannot diagnose an anovulatory cycle.

A Strong Rise Does Not Prove Ovulation Either

The reverse is equally important.

A sustained rise may be consistent with a post-ovulation progesterone pattern.

It remains indirect evidence.

Consumer temperature data does not directly measure:

• LH
• Progesterone
• Follicle rupture
• Egg release

Temperature and HRV Can Move Together During the Luteal Phase

Some wearable studies show that menstrual-cycle phase can coincide with changes in several physiological signals.

A possible luteal pattern can include:

• Higher finger skin temperature
• Slightly higher heart rate
• Lower HRV in some individuals

The size and direction of those changes vary substantially.

See our guide to menstrual-cycle changes in HRV and recovery for more context.

Do Not Require Every Metric to Change Together

You may show a clear temperature rhythm without a noticeable change in:

• Sleep duration
• Sleep efficiency
• Subjective recovery
• HRV

Research using continuous wearable measurements has found clear cycle-related finger-temperature changes even when several sleep outcomes remained stable.

One physiological signal does not dictate every other metric.

How RingConn Can Help Track Luteal Temperature Trends

RingConn records finger skin-temperature trends during normal wear and sleep.

The RingConn App can combine cycle history and temperature trends to provide estimated menstrual and cycle-phase information.

The most useful role is longitudinal:

one night provides a data point; multiple nights create a phase pattern; multiple cycles create a personal reference.

What RingConn Can Help You Observe

You may be able to observe questions such as:

• Does a sustained temperature rise appear around a similar phase each cycle?
• How long does my higher-temperature phase usually last?
• Does temperature move downward near menstruation?
• Does the curve look different during travel, illness, or unusual stress?
• How do HRV, heart rate, and sleep compare across cycle phases?

What RingConn Cannot Confirm

RingConn temperature and cycle information cannot clinically confirm:

• The exact day of ovulation
• Whether ovulation definitely occurred
• Whether the fertile window has ended
• Pregnancy
• Anovulation
• A hormonal disorder
• Fertility or infertility

Its cycle predictions and temperature information should be interpreted as wellness and cycle-awareness estimates.

How RingConn Gen 3 Fits Into Cycle Tracking

RingConn Gen 3 supports continuous wellness tracking including skin temperature, heart rate, HRV, sleep, SpO2, respiratory rate, activity, and other supported signals.

When combined with menstrual-cycle history in the RingConn App, these trends can provide broader context around your monthly pattern.

Users interested in continuous cycle and wellness trends can explore RingConn Gen 3.

A Practical Luteal Temperature Review

At the end of each cycle, ask five questions.

1. Was There a Sustained Shift?

Look for several higher nights rather than one spike.

2. When Did It Appear?

Compare the timing with your cycle history.

3. How Long Did It Last?

Measure the approximate higher-temperature phase rather than assuming 14 days.

4. Did Temperature Reset Around Menstruation?

Look for the broader downward transition.

5. Did Anything Confound the Cycle?

Review travel, illness, alcohol, stress, exercise, and environmental changes.

Example: A Clear Luteal-Like Trend

Phase	Possible Observation
Early cycle	Relatively stable lower temperature
Mid-cycle	Transition begins
Following nights	Sustained higher pattern
Approximately 12–14 days later	Temperature begins moving lower
Menstruation	Returns toward earlier-cycle range

This pattern can be consistent with an ovulatory cycle, but it does not independently prove ovulation.

Example: A Noisy Curve

Another cycle may contain:

• Three nights of travel
• A hard training session
• Alcohol
• One night of poor ring contact
• A respiratory illness

The resulting graph may not show a clean transition.

The right response is to document the context and keep tracking rather than forcing the curve into a textbook interpretation.

Example: A Later Temperature Rise

Suppose your previous cycles showed a sustained rise around cycle day 15.

This month it appears around day 20.

One plausible explanation is later ovulation.

That does not mean the luteal phase itself will necessarily become five days longer.

Follow the temperature pattern through the next period before drawing conclusions.

What If Temperature Stays Elevated Past Your Expected Period?

Pregnancy is one possible explanation for a sustained post-ovulation temperature pattern when menstruation does not occur.

Temperature cannot confirm pregnancy.

If pregnancy is possible and your period is late, use an appropriate pregnancy test rather than relying on wearable or BBT data.

Do Not Use an “18 High Days” Rule as a Pregnancy Test

Some traditional fertility charts describe prolonged high temperatures as an early pregnancy clue.

A sustained higher pattern can provide context, but:

• Ovulation timing may have been estimated incorrectly.
• Luteal length varies.
• Illness can raise temperature.
• Wearable skin temperature is not BBT.

An appropriate pregnancy test is the more direct method.

When Luteal-Phase Patterns Deserve Medical Context

Consider discussing your cycle with a healthcare professional when you experience:

• Repeated very short intervals between suspected ovulation and menstruation
• Persistent irregular or missed periods
• Very heavy bleeding
• Severe pelvic pain
• Repeated unexplained spotting
• Difficulty conceiving when pregnancy is desired

A temperature chart can provide useful history for that conversation but cannot diagnose the cause.

Luteal Temperature Tracking Checklist

• Start with an earlier-cycle personal baseline.
• Look for a sustained post-ovulation-like rise rather than one high reading.
• Expect luteal-phase length to vary rather than remain exactly 14 days.
• Compare temperature across several nights.
• Compare several menstrual cycles when possible.
• Remember that finger skin temperature is different from BBT and core temperature.
• Account for room temperature, alcohol, exercise, illness, stress, travel, and measurement continuity.
• Use temperature primarily as retrospective cycle context.
• Do not use a single temperature point to confirm ovulation.
• Do not use temperature alone as contraception.
• Use an appropriate pregnancy test when pregnancy is possible.

Final Takeaway

The luteal-phase temperature rise is one of the clearest recurring physiological patterns associated with an ovulatory menstrual cycle.

The mechanism is straightforward:

Ovulation → Corpus Luteum → Progesterone → Higher Temperature Pattern

The real-world curve is more complicated.

A useful interpretation framework is:

Follicular Baseline → Sustained Shift → Higher Luteal Pattern → Premenstrual Reset → Multi-Cycle Comparison

Do not focus on one perfect number.

There is no universal rule stating that your skin temperature must rise by 0.3°C, 0.5°C, or any other fixed amount.

There is also no requirement that the luteal phase last exactly 14 days.

The strongest information comes from a sustained change that appears in a biologically plausible phase, persists for multiple nights, and repeats across several cycles.

Temperature also has a major timing limitation: the clearest rise typically follows ovulation. This makes it more useful for recognizing a possible post-ovulation phase retrospectively than for predicting the fertile window in advance.

Finally, remember that RingConn tracks finger skin temperature trends. Skin temperature is different from core body temperature and traditional BBT, and one reading cannot confirm ovulation, pregnancy, fertility, or contraceptive safety.

Use the trend to understand your personal cycle pattern, combine it with accurate period history and broader context, and use appropriate reproductive or medical testing when a definitive answer is needed.

RingConn products are intended for personal health and wellness awareness and are not medical devices. Finger skin temperature, menstrual-cycle predictions, heart rate, HRV, sleep, and other RingConn wellness information are not intended to diagnose ovulation, anovulation, infertility, hormonal disorders, or pregnancy and should not replace professional medical advice, fertility evaluation, established contraception, pregnancy testing, diagnosis, or treatment.

FAQ: Luteal Phase Temperature Rise

How much does temperature rise during the luteal phase?

Core-temperature research commonly reports an average increase of approximately 0.3–0.7°C after ovulation, while BBT and finger skin-temperature studies report somewhat different values. There is no required skin-temperature increase for an individual wearable user, so focus on your sustained personal trend.

How many days does luteal temperature stay high?

Temperature commonly remains relatively higher through much of the luteal phase. The phase averages around 12–14 days, but meaningful individual and cycle-to-cycle variation occurs, and approximately 10–17 days can fall within broad clinical references.

Does temperature rise before or after ovulation?

The sustained rise is primarily associated with progesterone after ovulation. This makes temperature more useful for recognizing a possible post-ovulation phase retrospectively than for predicting exactly when ovulation will happen.

Can one high temperature confirm ovulation?

No. A single high reading can reflect environment, exercise, alcohol, illness, stress, or normal variation. A sustained multi-night increase provides stronger cycle context but still cannot clinically confirm that ovulation occurred.

Can I use a temperature rise to know when I am no longer fertile?

Temperature alone should not be used to determine contraceptive safety. The fertile window begins before the post-ovulation temperature rise becomes clearly visible. If avoiding pregnancy is important, use an established contraceptive method or an appropriately taught fertility-awareness method.

Why does my luteal temperature rise look different every month?

Ovulation timing, luteal-phase length, hormones, sleep, stress, exercise, alcohol, illness, travel, room temperature, and measurement continuity can all change the curve. Compare the broader phase-level pattern across several cycles instead of expecting identical nightly values.

Is RingConn skin temperature the same as BBT?

No. BBT is a standardized resting-temperature measurement usually taken immediately after waking. RingConn tracks finger skin temperature trends during wear and sleep. The two measurements can show related menstrual patterns but are not interchangeable.

Does a sustained high temperature mean pregnancy?

No. Pregnancy can be associated with a prolonged post-ovulation temperature pattern, but temperature cannot confirm pregnancy. If your period is late and pregnancy is possible, use an appropriate pregnancy test.