Pregnancy Week Calculator

Gestational age is counted in completed weeks and days from the first day of the last menstrual period (LMP), following the same 280-day (40-week) convention used for due-date estimates. A pregnancy that began 90 days ago is 12 weeks and 6 days along, in the first trimester, since the first trimester runs through 13 weeks 6 days. This is an estimate for planning purposes, not a medical diagnosis.

Tip: “Copy with settings” shares a link that opens this calculator with your numbers already filled in.

How this calculator works

This calculator shows how far along a pregnancy is right now, in completed weeks and days, using either the first day of the last menstrual period (LMP) or an already-known due date as the starting point. It also shows the trimester and roughly how many weeks remain until the due date.

These results are estimates based on standard dating conventions, not a measurement of the pregnancy itself. Ultrasound dating from a healthcare provider is more accurate than a calculation from LMP alone, especially with irregular cycles. This tool is for general information and does not replace medical advice - always defer to your healthcare provider for anything related to your specific pregnancy.

How the week is calculated

Gestational age (days) = chosen date − first day of last menstrual periodCurrent week and day = gestational age in days, split into completed weeks and remaining daysDue date (when starting from LMP) = LMP + 280 days (the 40-week / Naegele's rule convention)When starting from a known due date instead, the LMP is inferred as due date − 280 days

This calculator does not ask for cycle length, so it assumes the standard 28-day-cycle convention behind the 280-day estimate. If your cycles are notably longer or shorter, or you have a dating ultrasound, treat the due date it produces as approximate.

Worked example: LMP January 1, checked on April 1

Days since LMP: January 1 to April 1 is 90 days (31 days in January + 28 in February in a non-leap year + 31 in March = 90).
90 ÷ 7 = 12 weeks, remainder 6 days — so the pregnancy is 12 weeks 6 days along.
12 weeks 6 days falls before the 14-week mark, so this is the first trimester.
Estimated due date: January 1 + 280 days = October 8 of the same year, which is 190 days after April 1 — about 27.1 weeks remaining.

Frequently asked questions

How is the current pregnancy week counted?

By convention, pregnancy is dated from the first day of the last menstrual period, not from conception - so the count already includes about two weeks before ovulation typically occurs. Week 1 begins on that first day.

What if I only know my due date, not my last period?

Choose the 'known due date' option. This calculator infers the LMP by subtracting 280 days from the due date, then calculates your current week the same way.

When does each trimester start and end?

This calculator uses the common convention: first trimester through 13 weeks 6 days, second trimester from 14 through 27 weeks 6 days, and third trimester from 28 weeks onward.

Why might my provider's week count differ slightly from this calculator?

Providers often adjust dating after an early ultrasound, which can shift the estimated due date by a few days to a week or more compared to a plain LMP calculation. Their measurement generally takes precedence.

Is 'weeks remaining' exact?

No - it is the time until the estimated due date, and only about 5% of babies are born on that exact date. Most arrive within roughly two weeks of it, in either direction.