Gestational age

Gestational age is the duration of pregnancy counted from the first day of the last menstrual period. It conventionally begins about two weeks before fertilization.

Last menstrual period: The first day of the most recent menstrual bleeding before pregnancy. It supplies the conventional starting date for calculating gestational age.
Estimated due date: The predicted date of childbirth, commonly calculated as 280 days after the last menstrual period. It converts gestational age into a projected date for delivery.
Trimester: One of the three conventional divisions of pregnancy, each spanning roughly three months. Trimester boundaries organize care around gestational weeks.
Conceptional age: The time elapsed since fertilization, commonly used as a synonym for embryonic age. It starts at fertilization rather than the last menstrual period.
Menstrual cycle: Recurring hormonal and reproductive changes that prepare the body for possible pregnancy. Gestational dating assumes a cycle pattern even when ovulation timing varies.
Naegele's rule: A method for estimating a due date by adding 280 days to the last menstrual period. It is the standard arithmetic behind a common gestational-age estimate.
Prenatal screening: Tests offered during pregnancy to estimate the chance of fetal conditions. Screening windows are scheduled according to gestational age.
Fetal age: The developmental age of a fetus, often counted from fertilization. It can be confused with gestational age despite using a different starting point.
Ovulation: The release of an egg from an ovary, usually around the middle of a menstrual cycle. It typically occurs about two weeks after the date used to start gestational age.
First-trimester ultrasound: An early pregnancy ultrasound used to assess pregnancy location, viability, and embryo size. Crown–rump length can refine dating when menstrual dates are uncertain.

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