Pregnancy Week Calculator

Standard obstetric practice counts gestational age from the first day of the last menstrual period (LMP), not from conception. Naegele's rule sets the estimated due date at LMP plus 280 days (40 weeks), assuming a 28-day cycle with ovulation on day 14. Conception happens roughly 2 weeks after LMP, so a pregnancy reported as 8 weeks is about 6 weeks past conception. This calculator uses the same LMP baseline whether you start from your LMP, due date, conception date, or ultrasound gestational age, so the resulting week count is directly comparable to what your provider will use.

Calendar months range from 28 to 31 days, so 9 months can mean anywhere from 252 to 279 days. Obstetric care uses 280 days (40 weeks) as the standard term length from LMP because it is unambiguous, fits the weekly schedule of prenatal visits, and matches the trimester boundaries used in research and guidelines. Naegele's rule gives the same 280-day result by adding 7 days and 9 months to the LMP, but the weeks-and-days notation (for example 32 weeks 4 days) is what hospitals and most apps actually use day to day.

ACOG and SMFM both recommend that first-trimester ultrasound dating (before 14 weeks) replaces LMP dating whenever the difference is more than about 5 to 7 days. Crown-rump length measured between 7 and 14 weeks is accurate to within 5 to 7 days, while LMP dating depends on recall and assumes regular 28-day cycles with day-14 ovulation. The ultrasound mode in this calculator lets you enter the gestational age your provider read off the scan plus the scan date, so the entire week count and due date track that scan instead of an uncertain LMP.

This calculator uses the common clinical convention: the first trimester runs from 0 through 13 completed weeks, the second from 14 through 27 weeks, and the third starts at 28 weeks. Other sources split slightly differently, but the boundaries used here align with the rest of Calcoid's pregnancy calculators. The trimester label is informational; clinical decisions are made at specific week-level milestones, not on the trimester boundary itself.

Prenatal milestones are approximate and depend on dating accuracy and clinical circumstances. A first-trimester ultrasound can refine dating, an anatomy survey is commonly performed around 18 to 22 weeks, and standard term labels begin at 37 weeks. Viability is not a single cutoff at week 24. ACOG describes a periviable range from 20 weeks through 25 weeks 6 days in which outcomes change substantially with each week and with available neonatal care. Use your clinician's dated pregnancy record for decisions.