Pregnancy Due Date Calculator – LMP, IVF & Conception

Q: How accurate is an estimated due date?

An estimated due date is exactly that, an estimate. Only about 5% of babies are born on their exact due date. Most babies arrive within a 2-week window either side of the EDD. A first-trimester ultrasound before Week 14 is the most accurate method, with a margin of error of just 5-7 days.

Q: Can my due date change after an ultrasound?

Yes, and this is completely normal. If your first-trimester ultrasound crown-rump length differs significantly from your LMP calculation, your doctor may revise your due date. A difference of more than 7 days in the first trimester typically warrants an EDD revision.

Q: What is considered full term vs overdue?

Full term is 39-40 weeks. Early term is 37-38 weeks. Late term is 41 weeks, and post-term is 42+ weeks. Delivery before 37 weeks is preterm. Doctors typically discuss induction by 41-42 weeks to avoid risks associated with post-term pregnancy.

How It's Calculated

3 Ways to Calculate Your Due Date

Every pregnancy is unique. Depending on what information you have available, our calculator supports four scientifically validated methods to estimate your expected delivery date (EDD).

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LMP Method

Last Menstrual Period — Most Common

The standard clinical method used worldwide. Your due date is calculated as 280 days (40 weeks) from the first day of your last period — adjusted for your personal cycle length.

EDD = LMP + 280 days ± cycle adjustment

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IVF Transfer Method

Most Precise for IVF Pregnancies

For IVF pregnancies, the transfer date gives the most accurate EDD. The calculation accounts for the embryo's development stage — Day 3 (cleavage) or Day 5 (blastocyst) — at the time of transfer.

EDD = Transfer Date + 266 days − embryo age

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Conception Date Method

When You Know Ovulation Date

If you know your approximate conception or ovulation date — from tracking or an LH test — this method calculates from that point, adding exactly 266 days (38 weeks) to your conception date.

EDD = Conception Date + 266 days

Pregnancy Journey

Your Pregnancy Trimester by Trimester

A full-term pregnancy spans approximately 40 weeks from the last menstrual period, divided into three trimesters — each with distinct developmental milestones and maternal changes.

First Trimester

Weeks 1 – 13

Heart begins beating (Week 6)

Brain and spinal cord forming

Limb buds appear (Week 8)

All major organs present (Week 12)

First prenatal scan (Week 11–13)

Risk of miscarriage highest

Second Trimester

Weeks 14 – 27

Baby's movements felt (Week 18–20)

Anatomy scan (Week 20)

Hearing develops (Week 22)

Viability threshold (Week 24)

Glucose screening test

Most comfortable trimester

III

Third Trimester

Weeks 28 – 40+

Baby opens eyes (Week 28)

Rapid weight gain begins

Baby turns head-down (Week 34–36)

Lungs mature (Week 36)

Full term from Week 37

Birth preparation phase

Accuracy Guide

Which Method is Most Accurate?

Different calculation methods carry different levels of accuracy. Understanding which method best applies to your situation helps you interpret your estimated due date with appropriate confidence.

Method	Best For	Accuracy	Margin of Error	Confirmed By
Ultrasound (1st Trimester)	All pregnancies	⭐ Highest	± 5–7 days	Crown-rump length
IVF Embryo Transfer	IVF pregnancies	⭐ Very High	± 1–3 days	Exact transfer date
LMP (28-day cycle)	Regular cycles	✅ Good	± 7–14 days	Early ultrasound
Conception Date	Known ovulation	✅ Good	± 5–10 days	LH tracking/OPK
LMP (irregular cycle)	Irregular cycles	⚠️ Moderate	± 14–21 days	Early ultrasound essential

Fetal Development

Baby's Growth Week by Week

From a cluster of cells to a fully formed baby — here are the remarkable milestones your baby reaches across the key weeks of pregnancy:

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Week 6

Heartbeat

The tiny heart begins beating at approximately 100–160 bpm, visible on transvaginal ultrasound

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Week 8

Organs Forming

All major organs are present. Tiny fingers and toes begin to form. Baby is the size of a kidney bean

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Week 12

Brain Development

The brain develops rapidly. Baby can make facial expressions and reflexive movements

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Week 18–20

Movements Felt

Baby can hear sounds and may respond to your voice. First movements felt (quickening)

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Week 28

Eyes Open

Baby opens and closes their eyes. Rapid brain development begins. Considered viable outside womb

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Week 36

Lungs Mature

Lungs are nearly fully developed. Baby settles into head-down position in preparation for birth

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Week 37–38

Full Term

Baby is considered full term — fully developed and ready for life outside the womb at any moment

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Week 40

Due Date

Average time of delivery. Only 5% of babies are born exactly on their due date — a 2-week window is normal

Appointment Planner

Key Pregnancy Appointments to Book

Once you know your due date, use this guide to plan your essential prenatal appointments and screening tests throughout the 40 weeks of pregnancy.

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First Trimester (Weeks 1–13)

Foundation appointments

First prenatal booking appointment (Week 8–10)

Dating & viability ultrasound scan (Week 6–8)

NT scan & Down's syndrome screening (Week 11–13)

First blood tests (blood group, haemoglobin, infections)

Folic acid & prenatal vitamins prescribed

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Second Trimester (Weeks 14–27)

Anatomy & screening

Anomaly (anatomy) scan (Week 18–22)

Cervical length measurement if indicated

Gestational diabetes screening (Week 24–28)

Rhesus factor blood test if Rh-negative

Foetal growth monitoring scans if needed

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Third Trimester (Weeks 28–40)

Birth preparation

Growth and wellbeing scans (Week 28, 32, 36)

Group B Streptococcus (GBS) swab (Week 35–37)

Birth plan discussion with your doctor

Hospital registration and pre-admission

Antenatal (childbirth preparation) classes

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Post-Term Support (Week 40+)

If baby arrives late

Non-stress test (NST) to monitor foetal wellbeing

Biophysical profile ultrasound assessment

Induction of labour discussion (Week 41+)

Close monitoring of placental function

Only 5% of babies are born on their exact EDD

Expert Advice

Tips for a Healthy Pregnancy

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Start Folic Acid Early

Take 400–800 mcg of folic acid daily — ideally from 3 months before conception through the 12th week to prevent neural tube defects.

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Eat a Balanced Diet

Focus on iron, calcium, omega-3s, and protein. Avoid raw fish, unpasteurised dairy, and high-mercury fish throughout pregnancy.

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Stay Active Safely

Gentle walking, swimming, and prenatal yoga are excellent throughout pregnancy. Avoid contact sports and high-impact exercise after the first trimester.

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Prioritise Rest

Sleep on your left side from Week 28 onwards to optimise blood flow to the baby and placenta. Aim for 8–9 hours of quality sleep.

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Avoid Harmful Substances

No alcohol, smoking, or recreational drugs throughout pregnancy. Limit caffeine to under 200 mg per day (approximately one cup of coffee).

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Attend All Appointments

Never skip prenatal scans and blood tests — they detect complications early when intervention is most effective and least invasive.

FAQs

Frequently Asked Questions

How accurate is an estimated due date?

An estimated due date is exactly that — an estimate. Only about 5% of babies are born on their exact due date. Most babies arrive within a 2-week window either side of the EDD (between Week 38 and Week 42). The most accurate method is a first-trimester ultrasound (before Week 14), which typically has a margin of error of just 5–7 days. The LMP method assumes a standard 28-day cycle and may be less accurate for women with irregular cycles.

How is pregnancy counted — from conception or LMP?

Pregnancy is counted from the first day of your Last Menstrual Period (LMP) — not from conception. This is because ovulation and conception typically occur around 2 weeks after the start of your period, and most women don't know their exact conception date. This is why a 40-week pregnancy actually only involves approximately 38 weeks of actual foetal development.

Can my due date change after an ultrasound?

Yes — and this is completely normal. If your first-trimester ultrasound shows a crown-rump length (CRL) that differs significantly from what your LMP calculation suggested, your doctor may revise your due date to align with the ultrasound measurement. A difference of more than 7 days in the first trimester or 14 days in the second trimester typically warrants an EDD revision. The ultrasound date is generally more reliable than the LMP date.

How is an IVF due date calculated differently?

For IVF pregnancies, the due date is calculated from the embryo transfer date, accounting for the embryo's developmental age. For a Day 5 blastocyst transfer: EDD = Transfer Date + 261 days. For a Day 3 embryo transfer: EDD = Transfer Date + 263 days. IVF due dates are generally very accurate because the fertilisation date is known precisely — often more accurate than LMP-based calculations for natural pregnancies.

What is considered full term vs overdue?

Full term is defined as 39–40 weeks of pregnancy. Early term is 37–38 weeks (when baby is usually healthy but still maturing). Late term is 41 weeks, and post-term is 42+ weeks. Delivery before 37 weeks is considered preterm. Most doctors monitor pregnancies closely after 41 weeks and discuss induction of labour by 41–42 weeks to avoid risks associated with post-term pregnancy, including placental insufficiency.

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