Pregnancy Trimester Calculator (and Whose Cutoffs)

Trimester is a function of gestational age, so it starts with the same date everything else does. Enter your last period below — then read the section that explains why the answer depends on whose boundaries you use.

The cutoffs this page uses, and where they come from

Trimester	Gestational age
First	up to and including 13 weeks 6 days
Second	14 0/7 to 27 weeks 6 days
Third	28 0/7 weeks to delivery

These are the windows ACOG works in. Committee Opinion 700 defines the first trimester as “up to and including 13 6/7 weeks”, the second as “14 0/7 weeks to 27 6/7 weeks”, and the third as “28 0/7 weeks of gestation and beyond” — and its redating cutoffs are organized around exactly those edges. Since the practical use of knowing your trimester is understanding what your care team is scheduling, the calculator follows the boundaries your care team’s guidance uses.

The other division, which is not wrong

The Office on Women’s Health, part of HHS, publishes a different split on its stages-of-pregnancy page: first trimester weeks 1 to 12, second weeks 13 to 28, third weeks 29 to 40.

So at 13 weeks and 2 days you are in the first trimester by one federal-adjacent source and in the second by another. Neither is a mistake. A trimester is a communication device, not a physiological threshold — nothing switches on at 14 0/7 weeks. The OWH version divides a 40-week calendar into readable thirds for a general audience; the ACOG version aligns with windows that carry clinical decisions.

What matters is not picking a winner but knowing which one is on the page you are reading. The gap is roughly one week at each boundary, and it is enough to make two calculators disagree about the same pregnancy on the same day.

Worked example

Last period on January 1, today May 15:

• Gestational age: about 19 weeks
• Trimester: second, under either division — 19 is comfortably inside both

Now try January 1 and April 1: 89 days, which is 12 weeks and 5 days. First trimester by ACOG, first by OWH too. The disagreement only bites in the two narrow strips around weeks 13 and 28.

What each stretch is actually about

• First trimester. Organ systems form. MedlinePlus places the embryonic period at week 5, when major structures develop, and this is the stretch when exposures matter most for birth defects. It is also when the dating scan is most accurate.
• Second trimester. Growth and movement. Quickening — the first movements the mother can feel — falls around weeks 19 to 21, and by week 22 the baby’s heartbeat can be heard with a stethoscope. The anatomy scan sits at 18 to 20 weeks.
• Third trimester. Weight and lung maturity. The respiratory system produces surfactant from around weeks 27 to 30, and fat accumulates through weeks 31 to 34.

For the underlying week count, see the pregnancy weeks calculator; for the milestone-by-milestone version, pregnancy week by week.

This content is informational and does not replace prenatal care.