Pregnancy Weeks and Due Dates: How the Calculation Works

The 280 day rule with a real date

Pregnancy is measured in weeks counted from the first day of the last menstrual period, not from conception. That single convention explains most of the confusion around due dates: at the moment of conception you are already about two weeks pregnant by the medical clock. A full term pregnancy is 40 weeks, which is why the due date is the last period plus 280 days, a method known as the Naegele rule that most clinics still use as the starting estimate.

A concrete date makes it clear. Take a last period that started on 10 January 2026. Adding 280 days gives an estimated due date of 17 October 2026. If her cycle runs 30 days instead of 28, ovulation happened about two days later than the rule assumes, so the estimate moves two days later to 19 October. An early scan booked for 4 April would land exactly at 12 weeks and 0 days on this clock, since 12 weeks is 84 days after 10 January.

Everything else on this page refines that starting point: how cycle length adjusts the rule, what each trimester and landmark week contains, and why an ultrasound sometimes overrules the arithmetic.

The three trimesters at a glance

The 40 weeks divide into three blocks, and each has its own character: organ building, growth and refinement, then final growth and positioning for birth. The boundaries are drawn for medical convenience, not because anything switches off between weeks 13 and 14.

Week counts also translate into months only roughly, because calendar months hold four and a third weeks on average. A common mapping: month 1 is weeks 1 to 4, month 3 ends near week 13, month 6 near week 27, and month 9 runs to the due date. Use weeks for anything medical and keep months for conversation.

Note that the first trimester begins at week 1 even though conception happens around week 2 or 3. That is not an error; the convention anchors the whole timeline to a date everyone remembers, the first day of bleeding, rather than to an invisible ovulation.

The formula, including the cycle length adjustment

The base rule: due date = first day of last period + 280 days. On a 28 day cycle this assumes ovulation on day 14, leaving 266 days from conception to birth. The cycle adjustment follows directly: add one day to the due date for every day your cycle is longer than 28, and subtract one for every day shorter. Formally: due date = LMP + 280 + (cycle length - 28) days.

Two sanity checks show the size of the effect. A 35 day cycle pushes ovulation toward day 21, so the estimate moves about a week later; a 24 day cycle pulls it four days earlier. If cycles are irregular, the rule loses precision and early ultrasound takes over as the primary dating method. For IVF pregnancies the transfer date is known exactly: add 261 days for a day 5 blastocyst transfer, or 263 days for a day 3 embryo.

Adjustment in practice: write the cycle length next to the date you calculated, then shift it once. People who adjust twice, once for cycle length and once for ovulation pain or test dates, end up double counting; one adjustment is the rule.

Whichever route you use, the result is an estimate with a natural spread: most babies arrive between 37 and 42 weeks, and only around 4 to 5 percent appear on the date itself.

Landmark weeks worth knowing

A handful of weeks carry medical meaning beyond the trimester blocks. These are the dates clinics plan tests and visits around, so knowing them makes your appointment schedule readable.

Weeks 24 to 28 usually add the glucose tolerance test for gestational diabetes, and week 28 often starts third trimester visits every two weeks; anti-D injections for rhesus negative mothers also land around week 28. Your own schedule differs by country and by risk level, so treat the table as a map rather than an appointment book.

If you are reading this while already pregnant, the fastest way to find your current week is to count the days from the first day of the last period and divide by seven; the pregnancy calculator runs the same arithmetic instantly.

Why the ultrasound date sometimes wins

An early ultrasound measures the embryo from crown to rump, and between roughly weeks 7 and 14 this measurement predicts gestational age within a few days, often better than a remembered period. Periods are misremembered, ovulation moves around, and implantation timing varies, so scans before 14 weeks are the medical gold standard for dating. If the scan estimate differs from the LMP estimate by more than about 5 to 7 days, clinics redate the pregnancy to the scan.

After the second trimester the ranking reverses: babies grow at visibly different rates, so a late scan suggesting a different date usually reflects a bigger or smaller baby, not a wrong due date. Late third trimester scans should never be used to move the date. Bleeding in early pregnancy, irregular cycles and an uncertain last period all raise the value of an early scan even further.

One more nuance: IVF and early scans aside, the estimated date is always day based, not week based. Counting 280 days from a date is unambiguous, while counting 40 weeks can land a day off if someone rounds. Day arithmetic is what the tools and the clinics both use.

In practice the two sources merge: the LMP rule gives the first estimate, the first scan refines it, and the resulting obstetric estimate recorded in your notes becomes the working due date.

A full worked example from LMP to milestones

Put the whole method in one line of dates. A last period starts on 3 March 2026 on a 30 day cycle. Base due date: 3 March + 280 days = 8 December 2026. Cycle adjustment: 30 - 28 = 2 days later, so the adjusted estimate is 10 December 2026.

Milestones on that clock: 12 weeks lands on 26 May 2026 (3 March + 84 days), the 20 week anatomy scan around 21 July, week 24 viability on 18 August, and early term begins 17 November. At any later date, current week = days since 3 March divided by 7. On 1 September 2026 that is 182 days, exactly 26 weeks and 0 days, comfortably in the second trimester.

The arithmetic runs both ways: a scan on 18 August measuring exactly 24 weeks and 0 days points back 168 days to 3 March, recovering the last period. This is precisely the back and forth the pregnancy calculator and date difference tools automate for you.

Irregular cycles, IVF and other special cases

The rule assumes you know the first day of the last period and a roughly regular cycle. Irregular cycles, recently stopped hormonal contraception, breastfeeding at conception and PCOS all weaken that anchor. In those situations the early ultrasound becomes the primary estimate, and the LMP arithmetic turns into a cross check rather than the answer.

Assisted conception has exact anchors instead. IVF with a day 5 blastocyst: due date = transfer date + 261 days. A day 3 embryo: transfer date + 263 days. Frozen transfers use the thaw date the same way. Because embryo age is known exactly, dating scans are less likely to move these dates. Twins and multiples share one due date arithmetic, though they tend to deliver earlier, averaging around weeks 36 to 37.

Whatever the source of the date, write down which method produced it: last period with cycle length, a scan with a measurement, or an IVF transfer. When two dates disagree later in pregnancy, that note about provenance is what lets your care team decide which one to trust.