Pregnancy Week Calculator
See how many weeks and days pregnant you are from the first day of your last period.
Pregnancy weeks are counted from the first day of the last menstrual period, not from conception. This calculator’s main result is that week-and-day count. The due date is included so the week lines up with the other pregnancy tools.
Calculator Inputs
Used only to adjust the due date. Week count starts at the period date.
Results
Enter values above to see your results
These dates are estimates for planning, not a medical diagnosis. An ultrasound is more accurate, and only a small share of babies are born on the exact due date.
How to use this calculator
- Enter the first day of the last period. The sample date is 1 June 2026 so you can see a full result before you change it.
- Leave the cycle at 28 days unless your cycles are consistently shorter or longer. Cycle length moves the due date. It does not change how many days have passed since the period you entered.
- Read weeks and days first, then trimester and pregnancy month. Open the due date calculator if the date itself is the question you came with.
How the calculation works
Gestational age = today − last period, in days. Weeks are that number divided by 7, and the remainder is the extra days. “12 weeks and 3 days” means 87 days since the period started.
The due date uses Naegele’s count: 280 days after the period, plus or minus the difference between your cycle and 28 days. A 30-day cycle adds 2 days. The week you are today does not get that same adjustment, because the week is a count of days already elapsed.
Trimesters here switch at week 13 and week 28. Pregnancy months on the chart are unequal groups of weeks, not calendar months. Week 20 is month 5 on that chart (weeks 18–22), even though 20 ÷ 4.3 is closer to 4.6 calendar months.
Milestones by week
Week 12 is still the first trimester on this page (the second starts at week 13). Week 20 is halfway to a 40-week due date. Week 37 through 38 is early term, week 39 through 40 is full term, and week 41 is late term, using the American College of Obstetricians and Gynecologists definitions of those words.
A dating scan can move the official week. If a clinic gives you a different gestational age, use theirs for appointments. This page only does the arithmetic from the period date you type.
Worked example
Period on 1 June, checked on 1 September
1 Sep − 1 Jun= 92 days92 ÷ 7= 13 weeks, 1 dayWeek 13 or later= Second1 Jun + 280 days= 8 Mar 2027Week to trimester and pregnancy month
Month ranges follow the usual unequal pregnancy-month chart.
| Weeks | Trimester | Pregnancy month |
|---|---|---|
| 1–4 | First | 1 |
| 5–8 | First | 2 |
| 9–12 | First | 3 |
| 13–17 | Second | 3 then 4 |
| 18–22 | Second | 5 |
| 23–27 | Second | 6 |
| 28–31 | Third | 7 |
| 32–35 | Third | 8 |
| 36–40 | Third | 9 |
Frequently Asked Questions
How many weeks pregnant am I?
Count days from the first day of your last period and divide by 7. The leftover days are the “and X days” part. Conception is usually about two weeks after that period, so gestational age is about two weeks ahead of fetal age.
Does a longer cycle change my weeks?
Not the raw count from the period date. It does change the estimated due date. If you know ovulation was not on day 14, the ovulation due date calculator is the better dating tool.
When does the third trimester start?
At week 28 in this calculator. Week 27 is still the second trimester.
Guides & Articles
Due Date From Ovulation or Conception Date
Add 266 days to an ovulation date or a conception date. A worked example, why both dates use the same rule, and when the period method is the better start.
How Accurate Are Pregnancy Due Dates?
A due date is the 40-week mark from a period, or 38 weeks from ovulation. Most births are not on that exact day. What moves the estimate, and what replaces it.
How Many Days Is a Pregnancy?
280 days (40 weeks) from the last menstrual period, or 266 days (38 weeks) from conception. Same model, different starting point, plus the term labels clinicians use.
Related Calculators
Pregnancy Due Date Calculator
Estimate a due date from the first day of your last period
Pregnancy Weeks to Months
Convert pregnancy weeks and days into a pregnancy month and a trimester.
Due Date by Ovulation
Estimate a due date from an ovulation date or a conception date.
Ovulation Calculator
Track your fertility window and find your most fertile days to increase chances of conception