The three trimesters of pregnancy, week by week

Pregnancy is divided into three trimesters, each about three months long. The boundaries aren’t arbitrary — they mark real shifts in how a pregnancy develops and is cared for. Here’s where the lines fall, what weeks each covers, and how to find the exact dates for your own pregnancy with the trimester calculator.

How the trimesters are divided

The most widely used convention counts by gestational age from the last period:

• First trimester: 0 to 13 weeks
• Second trimester: 14 to 27 weeks
• Third trimester: 28 weeks to birth

Each is roughly 13 to 14 weeks. Some sources shift the exact cut-offs by a few days, but these are the boundaries used on this site and by many clinicians.

The first trimester (0–13 weeks)

The first trimester spans conception through the end of week 13. It’s a period of rapid early development and, for many, the most noticeable early symptoms. It’s also when a dating scan is most accurate, because early growth is so consistent. Dating established here tends to anchor the rest of the pregnancy.

The second trimester (14–27 weeks): when does it start?

The second trimester starts at 14 weeks. It’s often described as the more comfortable stretch, as early symptoms commonly ease. Key mid-pregnancy checks, including the anatomy scan discussed in how far along am I, fall in this window. Because the start depends on your dating, the calendar date differs for everyone — the calculator pinpoints yours.

The third trimester (28 weeks to birth): when does it start?

The third trimester starts at 28 weeks and runs until delivery, usually around 40 weeks. Monitoring often increases as the due date nears, and this is the window that contains the term milestones — early term at 37 weeks, full term at 39, and so on. Babies born before 37 weeks are preterm, which the World Health Organization highlights as an important driver of newborn health.

How long is each trimester?

Close to even thirds: the first is 13 weeks, the second 14 weeks, and the third about 12–14 weeks depending on when birth happens. Because a pregnancy runs about 40 weeks, three trimesters of ~13 weeks each fit neatly — which is exactly why the pregnancy is split this way. If you’re curious how those weeks translate into months, see pregnancy weeks to months.

Finding your trimester dates

To see which trimester you’re in today and the exact dates each begins and ends, enter your last period or due date into the trimester calculator. For your precise gestational age at any moment, use the gestational age calculator, and for the full picture with milestones, the OB wheel.