Pregnancy Weeks Calculator: How Many Weeks Am I

You are as many weeks pregnant as have been completed since the first day of your last period — not since conception, and not since the positive test. Enter that date to see the number.

Completed weeks: the distinction that causes every argument

Obstetric charts report completed weeks. At 12 weeks and 0 days, twelve weeks are finished and the thirteenth has just started. That is why your notes can say 12 weeks in the same conversation where your mother says you are in your thirteenth week, and neither of you is wrong.

A birthday works the same way and nobody trips over it: a person who turned 12 yesterday is 12, and is living through their thirteenth year. Pregnancy trips people up only because the units are small enough to notice.

Two practical consequences:

• The number turns over on a fixed weekday — the same day of the week as your last period, and therefore the same weekday as your due date. If the due date is a Thursday, you gain a week every Thursday.
• The extra days are not decoration. 12w6d and 13w0d are one day apart and land on opposite sides of the first-trimester boundary. Screening windows are written in weeks and days for exactly this reason.

Counting backward from a due date

If what you have is the due date rather than the period, the arithmetic runs in reverse. A pregnancy is 280 days at the due date, so:

Days completed = 280 − days remaining

With 100 days to go you are at 180 days, which is 25 weeks and 5 days. It is the same count from the other end, and it is worth knowing because a due date, once a scan has set it, is often the more reliable of the two numbers to work from — see gestational age by ultrasound.

Worked example

Last period on January 1, today February 26:

• Days elapsed: 56
• Weeks pregnant: 8 exactly, since 56 divided by 7 leaves no remainder
• Written on a chart: 8w0d

What the week number is for

The week is the unit US prenatal care runs on, and it is not interchangeable with a month. Screening tests have week-shaped windows. Term has week-shaped definitions: NICHD’s campaign exists because “term” once covered 37 through 42 weeks equally, and full term now starts at 39. Preterm begins below 37.

None of that survives translation into months, which is why the conversion lives on its own page rather than here — see pregnancy weeks to months if you want the month figure for a conversation, and keep the week for anything clinical.

For which trimester the week falls in, use the pregnancy trimester calculator; for what happens in each week, pregnancy week by week.

This content is informational and does not replace prenatal care.