Pregnancy Week Calculator

Gestational age counts from the first day of the last menstrual period (LMP), not from conception. Clinical care uses weeks + days (e.g., “8 weeks 3 days”). Pregnancy is typically dated as 40 weeks from LMP.

If you know your due date instead of LMP, gestational age can be derived: days pregnant = 280 − days until due date. Irregular cycles and IVF pregnancies need clinician-confirmed dating.

Example: LMP March 1, 2026, as of July 31, 2026 → about 21 weeks 5 days pregnant (first trimester complete). Switch to EDD mode if your provider gave you a due date of December 6, 2026 instead.

Gestational age and fetal age differ by about two weeks, and mixing them is the most common source of confusion. Clinical care counts gestational age from LMP, so at “8 weeks pregnant” the embryo has been developing for roughly 6 weeks. Apps and books that describe fetal development sometimes use the smaller number, which makes a pregnancy appear two weeks behind.

The weeks-plus-days notation is precise for a reason. “8 weeks 3 days” means 59 completed days, and screening windows are written in the same units because several tests are only valid inside a narrow range — cell-free DNA screening from about 10 weeks, and the nuchal translucency measurement between roughly 11 and 14 weeks.

Trimester boundaries are conventional rather than biological, which is why sources disagree slightly. A common division puts the first trimester through 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks onward; some clinics shift the first-second boundary to 12 or 14 weeks. Nothing changes physiologically at the line.

Viability thresholds are the other milestone people look for, and they have moved with neonatal care. Survival rises steeply between about 22 and 26 weeks, and outcomes depend heavily on the level of neonatal intensive care available — which is why any specific figure should come from your care team rather than a general calculator.

The two input modes are not equivalent in reliability. LMP mode reconstructs gestational age from your cycle and inherits all the uncertainty of that estimate; EDD mode works backward from a date your provider has already confirmed, usually with an early scan. If you have a clinician-confirmed due date, use it.

Ultrasound dating in the first trimester can shift gestational age by several days when LMP is uncertain.

Switch between LMP and EDD modes depending on whether you track from last period or an ultrasound-confirmed due date.