How the three trimesters are split
Pregnancy is divided into three trimesters, each roughly three months long and counted from the first day of your last menstrual period (LMP). The first trimester covers week 0 through 13 weeks 6 days, the second trimester runs from 14 weeks 0 days through 27 weeks 6 days, and the third trimester starts at 28 weeks 0 days and lasts until birth.
Because gestational age is measured from LMP rather than conception, you are counted as about 2 weeks pregnant at the moment of conception. That convention is what keeps these calculators, your doctor, and your ultrasound all using the same numbers.
Why a due date is only an estimate
The estimated due date is the single most likely day, but only about 1 in 20 babies actually arrive on it. Most births happen within two weeks on either side. If your cycle is longer or shorter than 28 days, set your average cycle length above so the due date and trimester ranges shift to match.
Frequently asked questions
When does each trimester start and end?
Counting from the first day of your last period (LMP): the first trimester runs from week 0 through 13 weeks 6 days, the second trimester from 14 weeks 0 days through 27 weeks 6 days, and the third trimester from 28 weeks 0 days until birth. In days from LMP that is roughly day 0-97, day 98-195, and day 196 onward.
How many weeks is each trimester?
The first trimester is about 14 weeks, the second is about 14 weeks, and the third is about 12 to 14 weeks depending on when you deliver. A full-term pregnancy is around 40 weeks from LMP, so the three trimesters together span those 40 weeks.
Which trimester am I in?
Find your gestational age in weeks from your last period, then match it to the boundaries: under 14 weeks is the first trimester, 14 to under 28 weeks is the second, and 28 weeks or more is the third. The calculator above does this for you and shows the exact dates.
How is gestational age counted (from LMP)?
Gestational age is counted from the first day of your last menstrual period (LMP), not from conception. Because ovulation happens about 2 weeks after LMP, you are considered 2 weeks pregnant before conception even occurs. This is the standard convention doctors and ultrasounds use, which is why the trimesters are measured from LMP.
Is my due date exact?
No. The estimated due date is a single best-guess day, but only about 1 in 20 babies arrive on it. Most births happen within two weeks either side of the due date. An early ultrasound (before week 13) dates a pregnancy more accurately than LMP alone, especially if your cycle is irregular.
Related calculators
- Pregnancy week calculator - see exactly how many weeks and days along you are.
- Due date calculator - estimate your delivery date from your last period.
- Pregnancy weeks to months calculator - convert your week count into months.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.