How your pregnancy due date is worked out

By Zulu Calculator · Updated 19 October 2026 · Health

Your due date is usually worked out by adding 280 days (40 weeks) to the first day of your last period. This is called Naegele's rule. If your last period started on 1 January 2026 and your cycle is 28 days, your estimated due date is 8 October 2026. A longer or shorter cycle moves the date by the same number of days, and an early dating scan can change it.

This guide explains each way a due date is worked out, how weeks and trimesters are counted, and when a scan date takes over. To get your own dates, use the pregnancy due date calculator. Whatever any calculator says, the due date your midwife or doctor gives you is the one to follow.

Why is pregnancy counted from my last period?

Because the first day of your last period is a date most people know, while the exact day of conception usually isn't. That day falls about two weeks before conception. A typical pregnancy lasts 40 weeks (280 days) from it.

This is why you are already "4 weeks pregnant" around the time you miss a period, even though conception happened only about two weeks earlier. Doctors, midwives and scans almost always count this way. It is called gestational age.

How is a due date calculated from my last period?

Start with the first day of your last period. Add 280 days. Then adjust for your cycle length, because ovulation usually happens later in a longer cycle and earlier in a shorter one.

due date = first day of last period + 280 days + (cycle length − 28) − (luteal phase − 14)

The luteal phase is the time from ovulation to your next period. It is usually 14 days, so for most people the last part is zero and can be left out. Only change it if you track ovulation.

Ovulation doesn't always happen on day 14. Stress, illness, travel and conditions such as PCOS can move it, so a date from your last period could be several days out. That is one reason an early scan is used to check it.

Due dates for different cycle lengths

All with a usual 14-day luteal phase. Each extra day of cycle adds one day to the due date.

How is a due date worked out from conception or IVF?

If you know when you conceived, you don't need your period date. Conception is counted as day 14 of the pregnancy and the due date as day 280, so the due date is always 266 days (38 weeks) after conception.

After IVF, the embryo's age is known exactly, so the transfer date pins down every other date. The eggs are fertilised on retrieval day, which counts as embryo day 0. A day-5 embryo is 5 days old when it goes in, so you take those 5 days off the 266.

Frozen embryo transfers and donor-egg cycles use the same maths. What matters is the embryo's age on transfer day, not when it was frozen or the age of the person carrying the pregnancy. ACOG Committee Opinion 700 says IVF due dates should use the embryo's age and the transfer date. For example, a day-5 transfer on 10 February 2026 gives 10 February + 261 days = 29 October 2026. A day-3 transfer on the same day gives 31 October, and a day-6 transfer gives 28 October. The conception and IVF due date calculator also covers IUI, egg retrieval and day-2 embryos.

How is a due date worked out from a scan?

A dating scan measures the baby and says how far along you were on the day of the scan, for example 11 weeks 1 day. Count on to 40 weeks from there.

due date = scan date + (280 − days pregnant at the scan)

An early scan, usually between 8 and 14 weeks, gives the most accurate due date, especially if your cycles are irregular or you're unsure of your dates. Up to 84 mm (about 13 weeks 6 days), the baby is measured from the top of the head to the bottom. This is the crown–rump length. After that, head, tummy and thigh-bone sizes are used. The ultrasound due date calculator works from any of these measurements.

When does a scan date replace my period date?

Only when the two dates are far enough apart. On the scan day, compare how far along the scan says you are with how far along your period date says you are. The American College of Obstetricians and Gynecologists (ACOG Committee Opinion 700, Methods for estimating the due date, May 2017) suggests changing to the scan's due date when the gap is more than:

The limits get wider later in pregnancy because later scans are less exact: babies grow at different rates. Some other points from the same guidance:

• The earliest good scan sets the date. Once a pregnancy is dated, later scans are used to check growth, not to change the due date.
• A pregnancy with no scan before 22 weeks 0 days is called "suboptimally dated".
• IVF pregnancies are dated from the embryo's age at transfer, not from a scan.

How are weeks and trimesters counted?

Once you have a due date, "day 0" is 280 days before it. Weeks pregnant are the whole weeks since day 0, plus the days left over.

day 0 = due date − 280 days
days pregnant = today − day 0
weeks pregnant = whole weeks in those days, plus the days left over
days to go = 280 − days pregnant

"10 weeks 0 days pregnant" means 10 whole weeks have passed, so you're in your 11th week.

Months are only a rough guide, because 40 weeks doesn't split evenly into 9 months. Doctors and midwives count in weeks. The pregnancy week calculator shows your week, trimester and the dates of each stage.

Worked examples

Example 1: last period on 1 January 2026, 28-day cycle

• Due date: 1 January 2026 + 280 days = 8 October 2026. The cycle is 28 days, so there is no adjustment.
• Likely conception: 1 January + 14 days = 15 January 2026.
• On 1 April 2026: 30 days (rest of January) + 28 (February) + 31 (March) + 1 = 90 days. 90 ÷ 7 = 12 with 6 left over, so 12 weeks 6 days, in the first trimester, with 280 − 90 = 190 days to go.
• Second trimester starts 9 April (14 weeks) and the third on 16 July (28 weeks).
• Full-term window: 17 September (37 weeks) to 21 October 2026 (41 weeks 6 days).

The same due date comes from a conception date of 15 January or a day-5 IVF transfer on 20 January.

Example 2: last period on 15 March 2026, 35-day cycle

• Cycle adjustment: 35 − 28 = +7 days.
• Due date: 15 March 2026 + 280 + 7 = 15 March + 287 days = 27 December 2026. With a 28-day cycle it would have been 20 December.
• Day 0 moves 7 days later too, to 22 March, and likely conception is 22 March + 14 = 5 April 2026.
• On 1 June 2026: 71 days since 22 March. 71 ÷ 7 = 10 with 1 left over, so 10 weeks 1 day, not the 11 weeks 1 day a 28-day count would give.

If you also knew your luteal phase was 12 days rather than 14, the due date would move 2 days later again: (35 − 28) − (12 − 14) = +9 days, giving 29 December 2026.

Example 3: a scan that changes the date

Your last period started on 1 January 2026, so your period due date is 8 October 2026. A scan on 28 March 2026 measures the baby at 11 weeks 1 day.

• Days pregnant by the scan: 11 × 7 + 1 = 78 days.
• Scan due date: 28 March + (280 − 78) = 28 March + 202 days = 16 October 2026.
• Days pregnant by your period on 28 March: 86 days (12 weeks 2 days).
• Gap: 78 − 86 = −8 days. The baby measures 8 days smaller.
• At 12 weeks 2 days the limit is 7 days. 8 is more than 7, so under ACOG's rule the scan date, 16 October 2026, is used.

If the same scan had said 12 weeks 0 days (84 days), the gap would be 2 days, under the limit, so you would keep 8 October.

How accurate is my due date?

It's an estimate. Only about 1 in 20 babies arrive on the due date itself. Most are born in the full-term window, between 37 weeks 0 days and 41 weeks 6 days.

If your baby hasn't arrived by around 41 weeks, your midwife or doctor will usually talk to you about your options.

Common mistakes

• Counting weeks from conception. Weeks pregnant count from day 0, about two weeks before conception. A scan or clinic's "6 weeks" means 6 weeks from day 0.
• Ignoring a long or short cycle. A 35-day cycle puts the due date a week later than a 28-day one.
• Using your period date after IVF. IVF due dates come from the transfer date and the embryo's age.
• Going back to the period date after a scan. Once a pregnancy has been dated by an early scan, that date is the one used from then on.
• Expecting later scans to move the date. After an early scan, later scans check growth instead.
• Adding 9 months. Months don't line up neatly with 40 weeks. Count 280 days.
• Treating the date as a deadline. Most babies arrive some time between 37 weeks and 41 weeks 6 days.

What if my dates don't match?

1. Use the date your midwife or doctor gives you. They set your official due date, usually from your early dating scan, and it is the one your care is planned around.
2. Check what went into each date. A different cycle length, a different first-day date or a scan can each explain a gap of a few days.
3. Ask at your next appointment if you are unsure which date to use. Bring your period date, cycle length and any scan report.
4. Don't wait for an appointment if you're worried. If you have pain, bleeding, fewer baby movements or any worry, contact your midwife, doctor or local emergency number straight away.

This guide is general information, not medical advice. A calculator can't tell you whether a pregnancy is healthy.

Quick summary

• Due date = first day of last period + 280 days (Naegele's rule).
• Move it one day later for each day your cycle is over 28 days, and one day earlier for each day under.
• From conception: + 266 days. From IVF transfer: + 263 (day 3), + 261 (day 5) or + 260 (day 6).
• An early scan is the most accurate; it replaces the period date when the gap is over ACOG's limit for that stage.
• Trimesters: to 13w 6d, 14w 0d to 27w 6d, then 28w 0d to birth. Full term is 37w 0d to 41w 6d.
• Only about 1 in 20 babies arrive on the due date.

Work out your dates with the due date calculator, see how far along you are with the pregnancy week calculator, check a scan with the ultrasound due date calculator, or count back to conception with the conception calculator.

Dating rules checked against ACOG Committee Opinion 700 (Methods for estimating the due date, May 2017), as used by our calculators (checked 9–11 October 2026).

This guide explains the rules in plain English for planning. It is not legal, financial or flight-operations advice; check your contract, the official source linked above, your employer or your operator.