Calculate pregnancy week from the last period

Clinical gestational age is counted from the first day of the last period, not from conception. Civil days are divided by 7: 10+3 means 10 weeks and 3 days. The first trimester runs through 13+6, the second through 27+6, the third after that. An ultrasound due date, if you have one, takes priority over this calendar.

days = date − LMP

weeks = ⌊days / 7⌋

Same origin as Naegele: 40 weeks of amenorrhea. We do not adjust for a 35-day cycle. Past 300 days the result is rejected: a pregnancy dated that way is no longer in the model.

First day of last period

First day of the last period.

As-of date (optional)

Date on which to read the age. Blank = today UTC.

Assumptions

• Dating from LMP.
• 280 days for term.
• Trimesters at weeks 1–13, 14–27, 28+.

Limits

• No biometry.
• No twins.
• Not medical advice.

People often say 12 weeks for anything from 12+0 through 12+6. The figure shown splits weeks and days to avoid that blur. Real follow-up uses the date the clinician kept.

• Counting from conception.

  Obstetric age starts from the period, about two weeks earlier.

• Rounding 12+4 up to “13 weeks”.

  You say 12+4, not the next week.

• Ignoring an ultrasound due date.

  If it redated the pregnancy, that is the date that counts.

• Ovulation calculatorOne date: the first day of the last period. The tool places ovulation on day 14 and a fertile window from day 10 to day 17.
• Pregnancy due dateOne date: the first day of the last period. The tool adds 280 days for the due date and 14 days for a model conception.
• Date differenceThe main result is a decomposition: 2 years, 5 months, and 10 days, for example. Total days sit beside it. That is not the same need as “how many stay nights.”

Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.

This calculation uses a standard mathematical identity. See also the methodology.