Pregnancy dates and weeks: what the numbers mean

Gestational age conventionally begins on day one of the last menstrual period, even though conception usually occurs later. This provides an observable date in a process where ovulation and fertilization are often not known exactly.

The classic formula assumes a 28-day cycle with ovulation around day 14. Irregular cycles, uncertain recall, unusual bleeding, or later ovulation can make an LMP estimate less reliable. Gestational dating is a convention; it does not mean a person was biologically pregnant on the first day of that period.

ACOG says first-trimester ultrasound, through 13 weeks and 6 days, is the most accurate method to establish or confirm gestational age. Crown–rump length dating in this period has an accuracy of about plus or minus five to seven days.

For pregnancies resulting from assisted reproductive technology, the ART-derived age should be used. Once LMP and the first accurate ultrasound are available, the EDD should be discussed and documented; later changes should be rare and clinician-led.

A gestational age such as 8 weeks + 3 days means eight complete weeks and three additional days. In everyday speech, somebody may call this “week nine,” but the clinical notation is less ambiguous.

Trimester boundaries use that same week-plus-day system. ACOG describes the first trimester through 13+6, the second from 14+0 through 27+6, and the third from 28+0 onward.

Current obstetric terminology separates early term, full term, late term, and postterm. ACOG defines early term as 37+0 through 38+6, full term as 39+0 through 40+6, late term as 41+0 through 41+6, and postterm as 42+0 and beyond.

These labels describe gestational periods; they are not instructions about an individual delivery. Clinical decisions depend on the pregnancy and belong with the maternity care team.

A calculator can apply a stated formula consistently, show milestone dates, and translate an established EDD into weeks plus days. It cannot examine the pregnancy or decide whether the input date was reliable.

Use the result to prepare for a care conversation or orient yourself before the first appointment. Once your midwife or doctor establishes an EDD, use that clinical date rather than repeatedly redating the pregnancy online.