Pregnancy Week-by-Week Calculator
See which week of pregnancy you are in right now and which trimester that falls in. Pregnancy is commonly tracked week by week, and people usually refer to the week they are 'in' — for example, being in week 14 once 13 full weeks have passed. Enter your last period date and the calculator tells you your current week, your completed weeks, and your trimester.
Calculate
Default result: 14
Pregnancy Week-by-Week Calculator · Result
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Week you are in
2026-01-01 × 2026-04-02
- Completed weeks
- 13
- Days into this week
- 0
- Trimester
- 1
This calculator provides an estimate for general information only and is not medical advice. Due dates, gestational age, and fertility windows are estimates — babies rarely arrive exactly on the estimated due date, and individual cycles and pregnancies vary. Always confirm dates and any health decisions with your healthcare provider or OB-GYN.
Reviewed by the calculators.dev team · Last updated 2026-06-24
Formula reviewed against Gestational age and trimester boundaries (review), PubMed 33079400; ACOG
How to calculate
Enter the first day of your last menstrual period and the date you want to check (today by default). The calculator counts the full weeks that have elapsed, adds one to get the week you are currently in, and shows the matching trimester.
completed weeks = (as-of date − first day of last period) ÷ 7, rounded down. The week you are 'in' = completed weeks + 1, because the first seven days are week 1. Trimesters follow ACOG: weeks 0–13 first, 14–27 second, 28+ third.
Example calculation
From a last period on January 1, 2026, April 2, 2026 is 13 completed weeks and 0 days. With 13 full weeks behind you, you are currently in week 14 — still the first trimester. People usually say the week they are 'in' (14), which is one more than the completed-weeks figure (13).
- currentWeek
- 14
- completedWeeks
- 13
- dayOfWeek
- 0
- trimester
- 1
Assumptions
- Weeks are counted from the first day of the last menstrual period — the same obstetric convention used for gestational age.
- The 'week you are in' is one more than your completed weeks: after 13 full weeks you are in week 14. This matches how pregnancy weeks are usually spoken about.
- Trimesters use ACOG boundaries: weeks 0–13 (first), 14–27 (second), and 28 onward (third).
- The count assumes a regular 28-day cycle. An early ultrasound is more accurate when cycles are irregular.
- A date before the last period is not a valid pregnancy week and is returned as an error rather than a negative week.
Common mistakes
- Mixing up completed weeks with the week you are in. Thirteen completed weeks means you are in week 14 — the figures differ by one.
- Expecting the trimester to change mid-week. Trimester boundaries fall on whole weeks (14 and 28), so the change happens at the start of a week.
- Counting from conception instead of the last period. Pregnancy weeks are dated from the last period, about two weeks before conception.
Frequently asked questions
What week of pregnancy am I in?
Enter your last period date and today's date. The calculator counts your completed weeks and adds one to give the week you are currently in — for example, week 14 after 13 full weeks.
Why is the week I am 'in' one more than my completed weeks?
The first seven days are week 1, so once a full week has passed you are in week 2. After 13 completed weeks you are in your 14th week.
Which trimester am I in?
Weeks 0–13 are the first trimester, weeks 14–27 the second, and week 28 onward the third, using ACOG boundaries.
Is this the same as gestational age?
It uses the same day math but is framed week by week. Gestational age is usually written as completed weeks and days (13 weeks 0 days), while here you also see the week you are currently in (week 14).