Pregnancy Weeks

Calculates how many weeks pregnant you are and the due date, from the first day of your last menstrual period. Pregnancy is dated from the LMP, not conception: the due date is LMP plus 280 days — 40 weeks — the same convention clinics use, with the week count as of today or any reference date you set.

How to use

1. Enter the first day of your last period.
2. Leave the reference date empty for today, or set one to see the week count on a specific day.
3. Read the weeks-and-days figure and the estimated due date.
4. Note the week count for appointment scheduling — screening tests are booked by week.

When you actually need this

• Getting an initial week count and due date before the first prenatal visit.
• Translating "how far along?" into the weeks-and-days format clinics use.
• Anticipating the timing of week-specific screenings and checkups.
• Estimating maternity leave dates from the projected due date.

Why the count starts before conception

Dating from the LMP means the first two weeks of "pregnancy" precede conception itself — ovulation typically happens around week two. The convention persists because the LMP is a date women can actually name, while conception rarely is. The 280-day figure is Naegele's rule, an average: only a small fraction of births land on the due date, and most arrive within the two weeks either side. A first-trimester ultrasound measures the embryo directly and frequently revises the date, especially with irregular cycles — the clinical date then supersedes this calculation. Full term spans 39 to 40 weeks; the due date is a planning midpoint, not an appointment.

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FAQ

Which time zone does Pregnancy Weeks use?

It uses your device's local time zone and calendar settings, so the result matches the dates you see elsewhere on your computer or phone. Nothing is converted to UTC behind the scenes.

Does Pregnancy Weeks handle leap years correctly?

Yes. The calculation works on real calendar dates rather than a fixed 365-day year, so February 29th, month lengths and century leap-year rules are all applied correctly.

Is the start or end day included in the count?

The tool shows the elapsed days between the two dates. If you need an inclusive count — for example counting today as day one of a challenge — add one to the result.