How Many Weeks Pregnant Am I? Due Date Math

Written by the AgeGapCalc Team · Last verified: 2026-10-13

Pregnancy is measured in weeks from the first day of the last menstrual period, with a typical full term near 40 weeks.

Our calculator converts between a due date and current date to show how many weeks along you are.

Weeks From Last Period

Pregnancy is dated from the first day of the last menstrual period, not from conception, which is why the count starts about two weeks before conception actually occurs.

Due Date Backwards

A due date is a countdown of 40 weeks from that starting point, so weeks pregnant can equally be worked out backwards from the due date.

A Math Aid, Not Medical Advice

The calculator reproduces the standard arithmetic; it does not assess a pregnancy. Always follow the dates given by a clinician, which may be adjusted after a scan.

Why the Dating Convention Matters

Because the starting point is the last period rather than conception, being six weeks pregnant means about four weeks after conception. Knowing the convention prevents confusion when comparing dates from different sources.

Gestational age is the clinical term for this count, and it is the figure that appears on scan reports.

Common Confusions Explained

The most common confusion is comparing a weeks-pregnant figure with the number of weeks since conception. The clinical count runs about two weeks ahead, because it begins at the last period.

The second is treating 40 weeks as an exact due date. It is a reference point, and clinicians routinely revise it after an early scan.

For the arithmetic view, the days between dates calculator converts the span from your last period into exact weeks and leftover days.

A Worked Example: From Last Period to Weeks

Suppose the first day of the last period was 3 August 2026 and today is 13 October 2026. The elapsed span is 71 days; 71 ÷ 7 is 10 remainder 1, so the count reads 10 weeks and 1 day. Add the full 280 days to the start date and the projected due date is 10 May 2027 — which is simply the 40-week mark of the same count.

Notice what the example does not tell you: when conception happened, or when the baby will actually arrive. The 71 days measure a convention, not a body. Clinicians keep the convention because it is checkable — the first day of the last period is usually known, while ovulation is not — and then adjust the dating with the first ultrasound where needed.

Trimesters, in Weeks

The 40 weeks divide into three trimesters, and the boundaries are week-based because the week count is the one everyone tracks. The first trimester runs weeks 1 to 13, the second weeks 14 to 27, and the third runs from week 28 until birth. A count of 13 weeks 6 days falls in the first trimester by one common reading and the second by another — sources differ by a day or two at the seams, which is harmless as long as the same convention is used throughout a pregnancy.

Within those trimesters sit the appointment milestones that make the week count practical: a dating scan near 12 weeks, an anatomy scan near 20, and the label "early term" once week 37 arrives. Those anchors are why an answer like "10 weeks and 1 day" is more useful than "about two and a half months" — the schedule is written in weeks, so the countdown should be too.

Why Early Pregnancy Is Counted in Days Too

The first weeks are tracked in finer units because the changes are fast: early pregnancy blood tests are typically spaced 48 hours apart, and a scan at 6 weeks 0 days versus 6 weeks 4 days can differ in what it is able to see. This granularity is also why prenatal care never rounds to months — a "month" can be 4.2 or 4.6 weeks depending on which calendar months the span crosses.

The same precision argument explains the arithmetic trap at the end: 40 weeks is 280 days, but ten calendar months is 304 or 305 days. Saying "nine months pregnant" at week 40 is not wrong, exactly — it is just a different, coarser ruler. When precision matters, count weeks and days from the last period; the weeks between two dates guide shows the division step by step, and the age-in-weeks page keeps the same convention going after birth.

Counting Backwards From a Due Date

The same count works backwards, and it is the only route when the last period is unknown but a due date exists. From a due date of 10 May 2027, count the days still to run on any given day and subtract that from 280: on 13 October 2026 there are 209 days to go, and 280 − 209 = 71 — the same 71 days that produced 10 weeks and 1 day from the front. Forward and backward answers must always agree; if they do not, one of the two dates was mistyped, and re-checking both takes seconds.

Backwards counting has one honest caveat. When an early scan has moved the due date, the scan-based date is the better starting point, and counting back from it will not reproduce the count from the last period — that difference is the adjustment doing its job, not an arithmetic error. As a rule, trust whichever starting point the clinician wrote down most recently and use the other only for orientation. Everything on this page is arithmetic; the dates that end up in a maternity notes folder belong to the people qualified to write them there.

Frequently Asked Questions

Sources

• Gestational age
• Estimated date of delivery

Weeks-based counting works for pregnancies and ages alike — the free age gap calculator reports any span in weeks as well as years and months.

Further Reading

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