Gestational Age Calculator — Weeks and Days

Gestational age is how far along a pregnancy is, counted in weeks and days from the first day of your last menstrual period — not from conception. Enter that date below to see today’s gestational age, the trimester and the estimated due date.

The arithmetic, and the two weeks nobody expects

Days since the last menstrual period, divided by seven. Whole weeks go on the left of the result, the remainder on the right: 100 days is 14 weeks and 2 days, written 14w2d in a chart.

The starting line is the confusing part. Fertilization happens roughly two weeks after the first day of a period, so the clock is already running before there is anything to run for. MedlinePlus puts the convention plainly: gestational age is measured from the first day of the last menstrual cycle. The practical consequence is that a positive test taken the week a period is missed usually lands at about 4 weeks — a number that feels a fortnight too high until you know where the count began.

It also means the familiar 40 weeks contains only about 38 weeks of embryo and fetus. For the month-by-month version of the same timeline, see pregnancy weeks to months.

Three ways to date a pregnancy, and a rule for the ties

Prenatal care has one gestational age, not three. When methods disagree, ACOG Committee Opinion 700 — written jointly with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine — settles it with single-point cutoffs rather than clinical taste:

Method	Accuracy	Where it is used
Last menstrual period	assumes a 28-day cycle and ovulation on day 14	the starting estimate for every pregnancy
First-trimester ultrasound (crown-rump length)	plus or minus 5 to 7 days	confirms or replaces the period-based date
Embryo age plus transfer date (IVF)	the conception date is known exactly	overrides both

For pregnancies conceived through assisted reproduction, ACOG is unambiguous: the ART-derived gestational age is the one assigned. That case has its own page, IVF gestational age.

The redating cutoffs themselves are laid out in gestational age by ultrasound. The short version: the earlier the scan, the smaller the disagreement it takes to move your date, from 5 days before 9 0/7 weeks up to 21 days in the third trimester.

Worked example

Last period on January 1, today April 11:

• Days elapsed: 100
• Gestational age: 14 weeks and 2 days
• Trimester: second, which ACOG opens at 14 0/7 weeks
• Estimated due date: October 8, that is 280 days after January 1

Why the number runs everything else

Screening windows are windows because gestational age defines them: nuchal translucency at 11 to 14 weeks, the anatomy scan at 18 to 20, glucose screening at 26 to 28. Growth is judged against it. So is the decision about when a baby may be delivered — NICHD’s national campaign exists because “term” once meant anything from 37 to 42 weeks, and full term now starts at 39.

ACOG also flags what happens when dating is left too late: a pregnancy with no ultrasound confirming or revising the date before 22 0/7 weeks is considered suboptimally dated, and elective delivery is not offered in one.

From here, the two methods split into dating by last period and dating by ultrasound, and the week count has a page of its own at pregnancy weeks calculator.

This content is informational. The gestational age of record for your pregnancy is the one your prenatal care team assigns and documents.