Trimester Calculator

Related pregnancy tools: Due date · Pregnancy weeks · Trimester dates · IVF due date · Ovulation estimate · Conception estimate

This calculator uses 14 weeks for the second trimester and 28 weeks for the third. The third trimester continues until birth; the date shown is the estimated due date.

About this calculator

Pregnancy is commonly divided into three trimesters, each covering roughly three months. The first trimester runs from the start of pregnancy through week 13, the second from week 14 to week 27, and the third from week 28 until birth, usually around week 40. This calculator works out which trimester you are in today and the calendar dates each trimester begins and ends. It accepts four starting points, the last menstrual period, a conception date, an IVF transfer date or a dating ultrasound, and converts each to the same underlying timeline.

The trimester boundaries are a convention, and different sources draw the lines a week or so apart. The split into weeks 0 to 13, 14 to 27 and 28 onward used here is a common one. Each trimester has its own broad pattern of development and of how a pregnancy is monitored. The dates this tool gives are estimates that move if the due date is revised, for example after a scan. For week-by-week detail, the NHS and the American College of Obstetricians and Gynecologists have guidance, and the pregnancy due date calculator works out the due date these dates are built on.

Sources checked 9 October 2026. These methods were checked against published guidance; this tool has not been clinically reviewed.

Frequently asked questions

What are the three trimesters of pregnancy?

Pregnancy is split into three trimesters, each about three months long. The first trimester covers the start of pregnancy through week 13, when the major organs form. The second, weeks 14 to 27, is often the most comfortable stage and includes the point at which movement is usually felt. The third, week 28 until birth, is when the baby gains most of its weight. The exact week each trimester starts and ends is a convention and varies slightly between sources.

Which trimester am I in?

It depends on how many weeks pregnant you are, counted from the first day of your last period. If you are up to 13 weeks and 6 days, you are in the first trimester. From 14 weeks to 27 weeks and 6 days, you are in the second. From 28 weeks onward, you are in the third. This calculator works your current week out from the date you enter and tells you the trimester, along with the dates each one begins and ends.

How long is each trimester?

Using this tool's convention, the first trimester ends at 13 weeks 6 days, the second runs from 14 weeks to 27 weeks 6 days, and the third starts at 28 weeks. The third trimester continues until birth; the calculator shows the estimated due date as a reference, not a fixed end.

When does the second trimester start?

By the boundaries this calculator uses, the second trimester starts at the beginning of week 14, which is 14 weeks and 0 days of pregnancy counted from the first day of the last period. The first trimester ends at 13 weeks and 6 days. Some sources place the change a few days earlier or later, so a chart that says week 13 is using a slightly different convention. The calculator shows the exact calendar date for your pregnancy.

Do trimester dates change after a dating scan?

They can. Trimester dates are built on the due date, and a dating ultrasound in early pregnancy can revise that due date, especially if it differs from the date worked out from the last period. If the due date moves, every trimester boundary moves with it. That is why this calculator lets you enter an ultrasound date and gestational age, so the trimester dates reflect the most accurate dating you have.

Last updated 2026-10-17