How accurate is the calculator?

It gives an estimated due date (EDD). Only about 5% of babies are born exactly on their due date — most arrive within 2 weeks either side.

Pregnancy is divided into 3 trimesters: 1st (weeks 1–12), 2nd (weeks 13–26), 3rd (weeks 27–40).

Why is pregnancy dated from my last period instead of conception?

Naegele's Rule dates from the LMP because it's the most reliably known date for most people — conception typically happens about 2 weeks later, which is already built into the 280-day calculation.

Can an ultrasound change my due date?

Yes — an early ultrasound (typically weeks 8-13) often gives a more accurate estimate than LMP-based dating alone, especially if your cycle isn't a standard 28 days. Your doctor may adjust the date after your first scan.

Why does the calculator use the first day of my last period?

It's the standard starting point because it's the easiest date to remember exactly. Conception usually happens about two weeks after that date, so the method is built around it. If you know your conception or IVF transfer date, those methods can be more precise.

Why do different due date calculators give different answers?

They start from different dates (last period, conception, or IVF transfer) and assume different cycle lengths or embryo ages. Use the calculator that matches the date you actually know.

What if I don't remember my last period?

Your care provider can estimate the due date from an early ultrasound, which is widely used when the period date is unknown or the cycle is irregular.

Is this calculator a substitute for medical advice?

No. It's a planning tool that gives an estimate. Confirm your due date and pregnancy progress with a doctor, midwife, or other qualified health professional.

Pregnancy Due Date Calculator

Calculate your pregnancy due date from last menstrual period, conception date, or IVF transfer date. Shows current week, trimester, and key milestones.

Calculation method

Last Period (LMP) date

How to Use

1
Choose calculation method
Select Last Period (LMP), Conception date, or IVF transfer date depending on what you know.
2
Enter the date
Pick the relevant date from the date picker.
3
View your due date
See your estimated due date, current week, trimester, progress percentage, and key milestones.

How It Works

This calculator estimates your due date using the same methods obstetricians use — Naegele's Rule from your last menstrual period, or a direct offset from a known conception or IVF transfer date — and shows your current week, trimester, and progress.

Naegele's Rule (from Last Menstrual Period)

Add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a standard 28-day cycle with ovulation on day 14 — pregnancy is conventionally dated from the LMP even though conception happens about two weeks later, which is why "40 weeks pregnant" doesn't mean 40 weeks since conception.

From a known conception or IVF date

If you know your conception date, the due date is conception + 266 days (38 weeks), since this skips the ~14-day gap between LMP and ovulation already built into Naegele's Rule. For IVF, the due date is calculated from the transfer date plus the known embryo age at transfer (e.g. a day-5 blastocyst transfer + 261 days) — generally the most precise of the three methods since the exact fertilization timing is known.

What the three trimesters mean for planning

The first trimester (weeks 1–12) is when most early symptoms appear: morning sickness, tiredness, breast tenderness, and frequent urination. Many people find nausea eases by the second trimester. The second trimester (weeks 13–26) often brings more energy, and the routine anatomy scan usually happens around weeks 18–22. The third trimester (weeks 27–40) is when the baby gains most of its weight, and practical preparations such as birth-plan discussions, hospital bags, and childbirth classes usually begin.

Why the due date is a target, not a deadline

A pregnancy is generally considered full term between 37 and 42 weeks. Because only around 5% of babies arrive on their due date, most families plan for a window of a few weeks around it rather than a single day. If you're past your due date, your care provider will usually monitor you more closely rather than assuming anything has gone wrong.

Examples

LMP-based estimate

If your last period started on 1 January, adding 280 days gives an estimated due date of 8 October — with the current trimester and week-by-week progress shown based on today's date.

IVF transfer-based estimate

For a day-5 blastocyst transferred on 1 March, the due date is 1 March + 261 days ≈ 16 November — typically more precise than LMP dating since the embryo's exact age is known at transfer.

Week 4 — about the size of a poppy seed

The embryo is only a few millimetres long and the neural tube, which becomes the brain and spinal cord, is starting to form. Many people don't know they're pregnant yet at this stage.

Week 8 — about the size of a raspberry

The major organs are forming. A heartbeat is often visible on an early ultrasound around this time, and the baby now has recognisable fingers and toes.

Week 12 — about the size of a lime

This marks the end of the first trimester. Many care providers consider the risk of early miscarriage lower after this point, and the baby can now move its limbs, though you won't feel it yet.

Week 16 — about the size of an avocado

The baby's face is fully formed and the skeleton is hardening. Some people begin to feel small flutters around this time, though first movements are more commonly felt between weeks 16 and 25.

Week 20 — about the size of a banana

This is roughly halfway through the pregnancy. The detailed anatomy scan is usually done between weeks 18 and 22, and it's also when the baby's sex can often be seen if you want to know.

Week 24 — about the size of an ear of corn

Week 24 is often cited as the threshold of viability, meaning a baby born this early may survive with intensive neonatal care. Survival and health outcomes improve considerably with each additional week.

Week 28 — about the size of an eggplant

The third trimester begins. The baby can open and close its eyes and is developing regular sleep and wake patterns. Your care provider may start talking to you about kick counts and warning signs.

Week 32 — about the size of a squash

The baby is gaining weight quickly and its lungs continue to mature. Many people find sleep harder in this stage and benefit from pillows for support.

Week 36 — about the size of a honeydew melon

Most babies have turned head-down by now, though some stay in other positions until labour. Your care provider will usually check the baby's position at appointments in the final weeks.

Week 40 — about the size of a small watermelon

This is the due-date week. Average newborns weigh around 3.2–3.5 kg, though the range is wide. Labour can start anywhere from 37 to 42 weeks, so it's normal for the birth to come earlier or later than this week.

Common Use Cases

Getting an early estimated due date to plan leave, appointments, and preparations
Tracking which trimester and week you're currently in
Cross-checking a due date given by LMP against a known conception or IVF date
Understanding pregnancy milestones tied to specific weeks (e.g. end of first trimester at week 12)

Tips

Only about 5% of babies are born exactly on their estimated due date — most arrive within about two weeks before or after, so treat the date as a planning target, not a prediction.
An early ultrasound (typically 8–13 weeks) often provides a more accurate due date than LMP-based dating alone, especially if your cycle isn't a standard 28 days — your doctor may adjust the date after your first scan.
The three trimesters are conventionally weeks 1–12, 13–26, and 27–40 — but exact boundaries can vary slightly by source, so don't be surprised by minor differences between calculators or care providers.
Cycle length matters for the LMP method. The estimate assumes a 28-day cycle with ovulation around day 14. If your cycles are consistently longer or shorter, ovulation happens earlier or later than that, and your provider may adjust the date.
If you're unsure of your last period date, say so — your provider can estimate the due date from an early ultrasound instead, which is often just as reliable.
Medical disclaimer: this calculator is for planning and general information only. It doesn't replace advice from a doctor, midwife, or other qualified health professional. Contact your care provider promptly about heavy bleeding, severe pain, fever, or — later in pregnancy — a noticeable drop in the baby's movements.

Frequently Asked Questions

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