How a Last Period Sets a Due Date

From a last period to a due date

Add 280 days to the first day of the last period when the cycle is 28 days. That is the Naegele count this page uses. For example, a last period on 1 January 2026 is due on 8 October 2026. Count the same period on 1 March 2026 and the page says 8 weeks and 3 days, still the first trimester, with 221 days left before 8 October.

Change the cycle and the due date, the days remaining, and the estimated conception date move together. For example, a 32 day cycle from that same January day is due on 12 October 2026, four days later, with 225 days remaining and conception on 19 January 2026. The week count stays 8 weeks and 3 days, because weeks are counted from the last period, not from the adjusted due date. The second trimester still starts 9 April 2026 and the third still starts 16 July 2026. The note under the date says the page used 32 and that the date moved four days later than a 28 day cycle. At 28 that note stays hidden.

A chat reply can recite "280 days." It will not keep the last period, the cycle length, the count date, and the trimester boundaries on one screen while you edit one of them. The arithmetic stays in the browser. Runs entirely on your device. Nothing is uploaded.

Why the 280 day count beats the month shortcut

The usual statement is one year minus three months plus seven days from the first day of the last period. What the Naegele rule counts, for a 28 day cycle, is about 280 days, and 280 days is what this page adds. The month shortcut and the day count disagree when the months in between do not add to 280 days. 31 March 2026 plus one year, minus three months, plus seven days is 7 January 2027. The same day plus 280 days is 5 January 2027. The page keeps the day count so a leap day and a short month cannot drift the result.

2026 is not a leap year, so 1 January plus 280 days is 8 October. For example, 1 January 2024 plus 280 days is 7 October 2024, because 2024 is a leap year and 29 February sits inside the span. The due date comes one calendar day earlier than the non-leap year. The page does not round that away.

Cycle length is the other input. The field accepts 21 to 45 days. Each day above 28 adds one day to the 280. Each day below 28 subtracts one. For example, a 21 day cycle from 1 January 2026 is due 1 October 2026, seven days earlier, and the note says seven days earlier. A 45 day cycle would move the date 17 days later. The week count and the trimester dates do not take that adjustment. They still start at the last period.

Reading the week count on any date

Subtract the last period from the date you are counting, in whole days, and split that span into weeks and leftover days. 1 January 2026 to 1 March 2026 is 59 days. 59 divided by 7 is 8 weeks, with 3 days left over. The card reads "8 weeks, 3 days." The same span with a 32 day cycle is still 8 weeks and 3 days. What changed is the due date, from 8 October to 12 October, and the days remaining, from 221 to 225.

Leave "Weeks pregnant on" blank and the page uses today. Fill it when you want a past or future day: a midwife appointment next Tuesday, or the day a test line appeared. The last period has to be on or before that date. A later period is rejected, because a negative week count would be a swapped pair of dates, not a pregnancy.

On the due date itself the count is 40 weeks exactly for a 28 day cycle, because 280 divided by 7 is 40. Two days later, 10 October 2026, the page says 2 days past due and 40 weeks and 2 days, in the third trimester. Past the due date is allowed. The date you type is the date the week count uses.

What the trimester cards and milestones show

This page starts the second trimester at 14 weeks, which is 98 days after the last period, and the third at 28 weeks, which is 196 days. From 1 January 2026 the second trimester starts on 9 April 2026 and the third starts on 16 July 2026. The due date for a 28 day cycle is 8 October 2026, which is inside the third trimester. Those two start dates are printed on the trimester card so you do not have to add 98 and 196 yourself.

The milestones under the result mark each of those dates reached or not, using the same count date as the week card. On 1 March 2026 none of them have arrived. On 10 October 2026 all three have. A 32 day cycle does not slide 9 April or 16 July. It only slides 8 October to 12 October. If you expected the trimesters to move with the due date, the card is telling you the page did not do that.

Estimated conception is a separate card. It is the cycle length minus 14 days after the last period. A 28 day cycle puts it on day 14, so 1 January 2026 gives 15 January 2026. A 32 day cycle gives 19 January 2026. The label says estimated. It is the same assumption a calendar method uses, and it is not a hormone test.

Last period dating versus an ultrasound scan

A date from the last period assumes ovulation about 14 days after it began. An early ultrasound measures the baby instead, so it does not need that assumption. When the two differ by more than set thresholds, which depend on how far along the scan is, the scan date replaces the last period date. That is the practice in ACOG Committee Opinion 700. Bring the date from this page to the scan, and use the date the clinician gives you after it.

If cycles vary by more than a few days, or with PCOS, a calendar date can be off by a week or more; an ultrasound or a clinician gives a better date. The cycle length field helps only when the cycle is long but steady. An unpredictable cycle has no single length to type.

Which mistakes move the date?

Sources

• Naegele's rule: 280 days, or 40 weeks, from the first day of the last period for a 28 day cycle.
• ACOG Committee Opinion 700, Methods for Estimating the Due Date: ultrasound dating replaces last period dating when they differ beyond set thresholds.
• NHS guidance on due dates and on when ovulation happens in a cycle.

What should you open next?

A pregnancy due date calculator versus the alternatives keeps the cycle length on the page and recomputes the due date, the weeks, and the trimester dates together. A span of calendar days that is not a pregnancy belongs in the date difference calculator. An exact age from a birth date belongs in the age calculator. Neither of those reads a cycle length, and this page does not try to compute an age in years.

Come back when the count date changes. The same last period and the same cycle produce a new week count in one edit. Leave the cycle at 28 only when 28 is the cycle you mean. A different length should be visible next to the due date, which is what the note is for.