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Pregnancy Trimester Calculator

Identify your trimester and weeks remaining from your week count.

How it is calculated

The tool takes the current gestational week (counted from the first day of the last menstrual period) and applies two cut-offs: weeks < 14 gives the first trimester, 14 ≤ weeks < 28 the second, and weeks ≥ 28 the third. Remaining time is 40 − weeks, never below zero, so it counts down to the conventional 40-week due date, not to the start of the next trimester. Decimals are accepted (13.9 is still the first trimester) and the remaining figure is rounded to a whole number of weeks, so 13.9 shows 26 weeks. Week 0 counts as the first trimester with 40 weeks left; a negative week gives no result. Weeks past 40 show the third trimester and 0 weeks left. Real due dates vary, and this is not medical advice.

Worked example

You are at week 20 of pregnancy. Since 20 is at least 14 but below 28, the result is the Second trimester. Remaining time is 40 − 20 = 20, so the tool shows 20 weeks until the conventional due date. To see how close you are to the third trimester, subtract the week from 28: here that is 8 weeks away, which the tool does not print itself.

Try this example →

Common mistakes

  • Entering the baby's age from conception. The week count here is gestational age from the last period, which is about two weeks ahead.
  • Reading the remaining weeks as the time left in the current trimester. The figure counts down to week 40, not to the next trimester boundary.
  • Expecting week 14 to still be first trimester. In this tool the second trimester starts exactly at 14, and the third exactly at 28.
  • Entering months instead of weeks. The field takes a week number, so type the week your clinician gives, not the month count.
  • Treating the 40-week figure as a due-date promise. Births can come earlier or later, and the tool shows 0 weeks for anything past 40.

Sources

About this calculator

A full-term pregnancy is conventionally divided into three trimesters based on gestational week: the first trimester runs from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 until birth, typically around week 40. Gestational age is counted from the first day of the last menstrual period, not from conception, which is why it runs about two weeks ahead of fetal age.

Obstetricians and midwives use trimester boundaries to structure prenatal care — timing screening tests like the first-trimester combined screen, the anatomy scan around 18-20 weeks, and the third-trimester glucose tolerance test — and expecting parents use them as a quick way to track which developmental stage and set of symptoms to expect. Each trimester carries its own typical milestones, from early morning sickness in the first to fetal movement in the second and pre-birth preparation in the third.

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