Pregnancy Trimester Calculator
Pregnancy is dated from the first day of the last menstrual period and split into three trimesters: the first through week 13, the second from week 14 to week 27, and the third from week 28 to birth around week 40. Enter your current gestational age in completed weeks and this calculator tells you which trimester you are in, how many days into pregnancy you are, and how many weeks remain to a 40-week term. It is general information; your provider may date your pregnancy slightly differently using an ultrasound.
Trimester boundaries
First trimester: weeks 1 through 13
Second trimester: weeks 14 through 27
Third trimester: weeks 28 to birth (around week 40)
Days into pregnancy = weeks * 7 + extra days
Percent of term = days into pregnancy / 280 * 100
Pregnancy is dated from the last menstrual period, making a full term about 40 weeks (280 days). Boundaries follow the standard obstetric convention.
Trimester context
- The first trimester covers weeks 1 to 13, when major organs form.
- The second trimester, weeks 14 to 27, is often when movement is first felt.
- The third trimester runs from week 28 to birth, typically around week 40.
- Term birth spans 37 weeks 0 days to 41 weeks 6 days; 39 to 40 weeks is full term.
- This tool is general information and not medical advice.
Pregnancy trimesters: frequently asked questions
How are the three trimesters of pregnancy defined?
Pregnancy is dated from the first day of the last menstrual period and divided into three trimesters. The first trimester runs from week 1 through the end of week 13, the second from week 14 through the end of week 27, and the third from week 28 to birth, typically around week 40.
How does this calculator find my trimester?
You enter your current gestational age in completed weeks. The calculator places you in the first trimester if that is 13 weeks or fewer, the second if 14 to 27, and the third if 28 or more. It also reports days into pregnancy and weeks remaining to a 40-week term.
Why is pregnancy dated from the last menstrual period?
Dating from the first day of the last menstrual period is the standard convention because that date is usually known, whereas the exact day of conception often is not. This makes a full-term pregnancy about 40 weeks from that start, even though fertilization happens roughly two weeks later.
Is 40 weeks the same as full term?
Forty weeks is the standard estimated due date, but full term is a range. Births from 37 weeks 0 days through 41 weeks 6 days are generally considered term, with 39 to 40 weeks regarded as full term. Your provider uses your individual dating for the most accurate estimate.
Is this calculator medical advice?
No. It performs calendar arithmetic on gestational weeks for general information. Your obstetric provider may date your pregnancy slightly differently using an ultrasound. Always rely on your care team for clinical decisions.
Official sources
- U.S. National Library of Medicine, MedlinePlus: stages of pregnancy.
- U.S. Centers for Disease Control and Prevention: Maternal and Infant Health.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.