Embryology · General embryology: weeks 1 to 8

Gestational age vs fertilization age: how pregnancy is dated !

Topic

Doctors count pregnancy from the last period; embryologists count from fertilization. The difference is about two weeks.

Updated 2026-11-01

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What it is

In clinical practice, gestational age is counted from the first day of the last menstrual period, because most women know that date. Fertilization happens about two weeks later, so embryonic (fertilization) age is about two weeks less. Embryology books use fertilization age; obstetric guidelines use gestational age. Mixing them up leads to errors when reading critical periods and ultrasounds.

Key facts

Gestational age = age since fertilization + about 2 weeks.
A term pregnancy lasts about 40 weeks of gestation, that is about 38 weeks after fertilization.
Preterm: born before 37 weeks' gestation; post-term: 42 weeks or later.
First-trimester ultrasound measuring crown–rump length is the most accurate way to date a pregnancy.
Naegele's rule: estimated due date = first day of the last period + 1 year − 3 months + 7 days (assumes a 28-day cycle).
Later in pregnancy ultrasound uses head, abdomen and femur, with a wider margin of error.

The same milestone on both clocks

Milestone	From fertilization	Gestational age (LMP)
Fertilization	Day 0	About 2 weeks
Implantation begins	Day 6	About 3 weeks
Neural tube closes	Day 28	About 6 weeks
End of the embryonic period	Week 8	Week 10
Term birth	About 38 weeks	About 40 weeks

Clinical notes

When reading a teratogen's critical period, check whether it is in weeks from fertilization or from the last period.
Irregular cycles, implantation bleeding or recent hormonal contraception make the last period unreliable; ultrasound then takes over.
A wrong gestational age can lead to misdiagnosing growth restriction, prolonged pregnancy or preterm labor.
In the newborn, gestational age is estimated with scores that examine physical and neuromuscular maturity.

Test yourself

Think of the answer, then tap the question to see it.

What is the difference between gestational age and fertilization age?

Gestational age is counted from the first day of the last menstrual period and is about two weeks more than fertilization age.

Which method dates a pregnancy most accurately?

First-trimester ultrasound measuring the crown–rump length.

If an embryology book says the neural tube closes around day 28, what gestational age is that?

About 6 weeks' gestation counted from the last menstrual period.

Below which gestational age is a newborn considered preterm?

Born before 37 weeks of gestation.

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Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 6: Third to Eighth Weeks: The Embryonic Period (Wolters Kluwer, 2015); Gestational age: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).

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