Pregnancy Trimester Calculator: What Week Are You In and What Comes Next?

The pregnancy trimester calculator tells you exactly which week of pregnancy you are in, which trimester that week belongs to, and when each trimester ends — find your estimated due date first with the Pregnancy Due Date Calculator.

Why Knowing Your Trimester Matters More Than Most People Realize

Most pregnant women know they are in "early pregnancy" or "late pregnancy" — but not knowing the exact week and trimester means missing time-sensitive screenings and appointments that are only offered in specific windows.

According to the American College of Obstetricians and Gynecologists, the number of recommended prenatal visits roughly doubles from the first trimester to the third — from 1 to 2 visits in the first trimester to 7 or more in the final 10 weeks. Each visit corresponds to a specific developmental stage with tests and screenings that cannot simply be rescheduled to any convenient time.

People who do not track their trimester precisely miss screening windows that close permanently. The nuchal translucency ultrasound — which screens for chromosomal conditions — is only available between weeks 11 and 14. Missing this window by even 1 week can result in $800 to $1,200 in additional testing to access equivalent information through later, more invasive methods.

Pregnancy Trimesters Explained in Plain English

Pregnancy is divided into three trimesters based on gestational age — the number of weeks since your last menstrual period, which is how doctors measure pregnancy length. The first trimester covers weeks 1 through 13. The second trimester covers weeks 14 through 27. The third trimester runs from week 28 through the end of pregnancy at week 40 or beyond.

This pregnancy weeks calculator finds where you sit right now by calculating the number of days since your last period and converting that into weeks and a trimester label. Knowing your trimester is not just a label — it is a scheduling guide. Different screenings, vaccines, supplements, and physical activity recommendations apply to each trimester, and your care team uses trimester to set the agenda at every prenatal visit.

The Pregnancy Trimester Formula — Step by Step

Trimester Formula: Gestational Week = (Today's Date − LMP) ÷ 7 | T1 = Weeks 1–13 | T2 = Weeks 14–27 | T3 = Weeks 28–40

Last Menstrual Period (LMP) is the anchor date — the first day of the period before this pregnancy began. Everything in pregnancy dating starts here. If your LMP was April 1 and today is June 10, the gap is 70 days. Using any date other than the confirmed first day of your last period introduces error that shifts your trimester assignment.

Gestational Week converts the day count into a week number. 70 days ÷ 7 = exactly 10 weeks. You are in week 10 — or more precisely, in the middle of week 10, heading toward week 11. Most sources describe pregnancy week by week, so having this number tells you exactly which developmental events to expect and which appointments to schedule this week.

Trimester Assignment places your week in one of three stages. Week 10 falls in the first trimester (weeks 1 through 13). Once you know your trimester, check how many weeks remain until your next trimester transition — at week 10 you have 3 more weeks before the second trimester begins at week 14. For week-by-week fetal development data matched to your current gestational age, use the Fetal Weight Calculator.

Worked Example: A First-Time Mother Checks Her Trimester at Her 8-Week Scan

Sofia is 31 and had her first prenatal appointment today. Her last menstrual period started February 3. The appointment date is March 31. She wants to know exactly which week and trimester she is in before her ultrasound.

Gap: March 31 − February 3 = 56 days. Gestational week: 56 ÷ 7 = 8.0, meaning Sofia is at the start of week 8. Week 8 falls in the first trimester (weeks 1–13).

Sofia is in week 8 of her first trimester with 5 weeks remaining until the second trimester begins at week 14. Her nuchal translucency screening window opens in 3 weeks and must be completed by week 14 — she has a specific 3-week booking window to act on.

Sofia immediately asks her midwife to schedule the nuchal translucency scan for weeks 11 to 12 — giving her time within the window without waiting too long. She also notes that her anatomy scan will be offered around week 20, which falls in mid-July.

What to Do with Your Pregnancy Trimester Calculator Result

• Run the trimester calculator above at every new week to track your exact position — many key tests are only available for a 2 to 4 week window, and knowing your week prevents missing the booking period.
• Schedule your nuchal translucency ultrasound at week 11 to 12 if you are currently in weeks 8 to 10. Counter-intuitively, the first trimester carries more time-sensitive screening than any other — waiting until the second trimester feeling "more established" causes people to miss tests that cannot be repeated.
• Use the Pregnancy Weight Gain Calculator once you know your trimester — recommended weight gain targets are different for each trimester, and your current trimester is the input that sets the specific range your midwife will discuss.
• Write your trimester transition dates on your phone calendar for weeks 14 and 28 — these are the dates your care schedule intensifies and your provider will begin offering trimester-specific advice and screenings.

Pregnancy Trimester: 5 Common Questions Answered

Q: How many weeks pregnant am I if my last period was 10 weeks ago? A: You are in week 10 of your first trimester. Count from the first day of that period to today in days, then divide by 7. Your first trimester ends at week 13, meaning you have approximately 3 weeks before entering the second trimester.

Q: When exactly does the second trimester start? A: The second trimester begins at the start of week 14 — when you have completed 13 full weeks of pregnancy. Some sources define it as week 13 or at 12 weeks and 6 days. The standard obstetric definition used in most prenatal care settings is week 14.

Q: Are the three trimesters equal in length? A: No — and this is a common misconception. The first trimester is 13 weeks, the second trimester is 14 weeks (weeks 14 to 27), and the third trimester is at least 13 weeks (weeks 28 to 40 or beyond). The second trimester is actually the longest of the three.

Q: Can I feel different symptoms depending on my trimester? A: Yes — symptoms are strongly trimester-dependent. Nausea and fatigue are most common in the first trimester and usually ease by week 12 to 14. Energy often returns in the second trimester. Back pain, Braxton Hicks contractions, and shortness of breath become more common in the third trimester.

Q: What are the trimester dates for a due date of December 18? A: Working backward from December 18: Last menstrual period is approximately March 13. First trimester: March 13 to June 12 (weeks 1–13). Second trimester: June 12 to September 18 (weeks 14–27). Third trimester: September 18 to December 18 (weeks 28–40). These are the trimester dates for that specific due date.

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Related: Pregnancy Due Date Calculator | Fetal Weight Calculator