Trimester Date Calculator
Pregnancy is conventionally divided into three trimesters, each marking a distinct stage of development. Dated from the first day of your last menstrual period (LMP), the first trimester runs through week 12, the second through week 26, and the third until birth around week 40. Enter your LMP below to see the calendar start and end date of each trimester plus your estimated due date. These are planning estimates based on a standard 40-week pregnancy. A dating ultrasound from your care provider gives the most accurate timeline.
Trimester date formula
EDD = LMP + 280 days (40 weeks, Naegele's rule)
First trimester = LMP to LMP + 90 days (end of week 12)
Second trimester = LMP + 91 days to LMP + 188 days (end of week 26)
Third trimester = LMP + 189 days to EDD
Week N completes at LMP plus (N times 7) days. The end of week 12 is therefore LMP plus 84 days at the start of week 13; this calculator reports each trimester boundary using completed weeks. Gestational age is always counted in completed weeks plus days from the LMP.
Worked example
Suppose your LMP was 1 January 2026.
- EDD = 1 January 2026 + 280 days = 8 October 2026.
- First trimester: 1 January through the end of week 12 (around 26 March 2026).
- Second trimester: start of week 13 (around 26 March) through the end of week 26 (around 2 July 2026).
- Third trimester: start of week 27 (around 2 July) through your due date of 8 October 2026.
Trimester dates: frequently asked questions
When does each trimester start and end?
Pregnancy is dated from the first day of the last menstrual period (LMP) and lasts about 40 weeks. The first trimester runs from week 0 through the end of week 12. The second trimester runs from week 13 through the end of week 26. The third trimester runs from week 27 until birth, around week 40. This calculator converts those week boundaries into calendar dates from the LMP you enter.
How is the due date calculated?
The estimated due date (EDD) is the LMP plus 280 days (40 weeks). This is Naegele's rule, the standard convention used in obstetrics. Actual delivery may occur any time from about 37 to 42 weeks. Only about 4 percent of births happen exactly on the due date.
Why is pregnancy dated from the last period and not conception?
Conception usually occurs around 2 weeks after the first day of the last menstrual period, but the LMP is a date most people can recall reliably while the exact conception date is rarely known. Dating from the LMP is the standard clinical convention, so a pregnancy is already counted as 2 weeks along at the moment of conception.
Are trimester boundaries exact?
No. Trimester boundaries are conventions used to group the stages of pregnancy, and clinicians use slightly different week cut-offs (for example some place the second trimester at weeks 14 to 27). This calculator uses the common 0 to 12, 13 to 26, 27 to 40 split. An ultrasound dating scan can adjust your dates if your cycle is irregular.
Official sources
- U.S. National Library of Medicine, MedlinePlus: Pregnancy.
- Office on Women's Health, U.S. Department of Health and Human Services: Pregnancy.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.