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Pregnancy Week by Week Calculator

Enter your last period or your due date, and the date you want to check. You get your week and day of pregnancy, your trimester, how many days are left and a calendar that lists the dates of all 40 weeks.

Your numbers

Result

Estimate for information only, not professional advice. Last reviewed .

How we calculate it

Pregnancy is dated from the first day of the last period and lasts about 40 weeks (280 days). If you know your due date instead, the start is the due date minus 280 days. Weeks are counted from that start, so a person who is “10 weeks 3 days” pregnant is 73 days past it.

The first trimester is weeks 1 to 13, the second is weeks 14 to 27 and the third starts at week 28. Doctors group the end of pregnancy as early term (37 to 38 weeks 6 days), full term (39 to 40 weeks 6 days), late term (41 weeks) and post term (42 weeks or more).

Assumptions and limits

  • This is an estimate for general information. It is not medical advice and cannot replace a conversation with a doctor or midwife. Only about 5% of babies are born on the due date.
  • If an early ultrasound gave a different due date, your doctor or midwife’s dating takes priority, so use the due date option.
  • This page gives dates and stages only. It does not predict baby size or health.
  • Sources: ACOG Committee Opinion 579, Definition of Term Pregnancy (early term 37 0/7 to 38 6/7, full term 39 0/7 to 40 6/7, late term 41 0/7 to 41 6/7, postterm 42 0/7 and beyond); ACOG Committee Opinion 700, Methods for Estimating the Due Date.

Frequently asked questions

How many weeks pregnant am I?

Count the days from the first day of your last period and divide by 7. The calculator does this for any date and also works backwards from a due date.

Why is pregnancy counted from the last period and not from conception?

Conception happens about two weeks after the period starts but the date is rarely known exactly, so doctors count from the period. That is why a pregnancy is 40 weeks but a baby grows for about 38.

When does the second trimester start?

At week 14, which is day 98 of the pregnancy. The third trimester starts at week 28, day 196.

What counts as full term?

Between 39 weeks and 40 weeks 6 days. Birth at 37 to 38 weeks 6 days is early term, and 41 weeks is late term.

Does the calculator work if my cycle is not 28 days?

Yes. Enter your usual cycle length and the dating moves by the difference from 28 days, the same adjustment as the due date calculator.

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Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: I know my = Last period; First day of your last period = January 10, 2026; Usual cycle length (days, 21 to 45) = 28; Date to check = March 21, 2026.

  • You are 10 weeks 0 days
  • Estimated due date Saturday, October 17, 2026
  • Trimester First trimester
  • Days to go 210
  • Progress 25%
  • Stage Preterm (before 37 weeks)
  • Second trimester starts Saturday, April 18, 2026
  • Third trimester starts Saturday, July 25, 2026
  • Full term (39 weeks) starts Saturday, October 10, 2026

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.