Pregnancy week calculator
Enter your last period, see how far along you are.
How many weeks pregnant am I?
Count from the first day of your last menstrual period: the number of full weeks and days since then is how far along you are. The calculator does it for any date.
Why does pregnancy count from the last period?
By convention, ACOG dates pregnancy from the first day of the last period, and pregnancy is assumed to start 2 weeks after it. A normal pregnancy lasts about 40 weeks from that day.
What do "8 weeks 3 days" and "week 9" mean?
They are the same point: 8 full weeks and 3 days are done, so you are in the 9th week.
When do the trimesters start?
The first trimester runs through 13 weeks 6 days, the second from 14 weeks through 27 weeks 6 days, and the third from 28 weeks.
What if I know my due date instead?
Choose "Due date": the calculator counts back 280 days to find the start and works out the weeks from there.
How are the weeks of a full-term pregnancy named?
ACOG calls 37 weeks through 38 weeks 6 days early term, 39 through 40 weeks 6 days full term, 41 through 41 weeks 6 days late term, and 42 weeks onward postterm. The names replaced the older single label of “term” beginning at 37 weeks.
A worked example
Suppose the first day of the last menstrual period was January 5, 2026. Adding 280 days gives an estimated due date of October 12, 2026. On March 16, 2026 that is 70 days since the start, which is 10 weeks 0 days, so you are in week 11. On April 22 it is 15 weeks 2 days. The trimester boundaries for this pregnancy fall on April 13, 2026 (14 weeks, the second trimester) and July 20, 2026 (28 weeks, the third), and 37 weeks is reached on September 21, 2026.
Why the calendar starts before conception
Doctors count from the first day of the last period because that date is usually known, while the date of conception is not. Ovulation is assumed to come about 14 days after the period begins, so a pregnancy dated this way is about two weeks older on paper than the baby. A “40-week” pregnancy is therefore about 38 weeks from conception. The same arithmetic runs backward when you enter a due date: the calculator subtracts 280 days to find the start.
Common mistakes
Counting from the date of a positive test or from conception makes you look about two weeks less far along than your provider’s chart. Treating “8 weeks 3 days” as week 8 is the next: that is the ninth week, though your chart will say 8 weeks. And treating the due date as an appointment is a third: it is an estimate, and ACOG describes births from 39 weeks to 40 weeks 6 days as full term (October 5, 2026 for the example above) and from 42 weeks (October 26, 2026) as postterm. If an early ultrasound gives a different date, your provider may revise the due date, and that revised date is the one to use.
Sources: American College of Obstetricians and Gynecologists, Committee Opinion No. 700 (Methods for Estimating the Due Date) for the 280-day count from the last period, and Committee Opinion No. 579 (Definition of Term Pregnancy) for the term labels. Related: due date calculator, conception calculator, ovulation calculator and pregnancy weight gain calculator.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.