[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general) › Gestational age vs fertilization age: how pregnancy is dated [Embryology](https://durgical.com/en/embryology/) · [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general) # 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 **See it in 3D:** [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) · [Fundus of uterus](https://durgical.com/en/anatomy/fundus-of-uterus/) ## Key points - Clinic: gestational age from the LMP. Embryology: age from fertilization. - The gap is about 2 weeks. - Term ≈ 40 weeks' gestation = 38 weeks after fertilization. - Most accurate dating: first-trimester crown–rump length. - Preterm - 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](https://durgical.com/en/conditions/preterm-labour-and-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. ## Memory aid The obstetrician's clock starts two weeks before the embryo's: it begins at the period, when the baby does not even exist yet. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/gestational-age-vs-fertilization-age-how-pregnancy-is-dated/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) - [Fundus of uterus](https://durgical.com/en/anatomy/fundus-of-uterus/) ## Related - [Weeks 3–8: the embryonic period and organogenesis](https://durgical.com/en/embryology/weeks-3-8-the-embryonic-period-and-organogenesis/) - [Week 1: cleavage, morula, blastocyst and implantation](https://durgical.com/en/embryology/week-1-cleavage-morula-blastocyst-and-implantation/) - [Ovarian cycle, ovulation and the corpus luteum](https://durgical.com/en/embryology/ovarian-cycle-ovulation-and-the-corpus-luteum/) - [Fetal period: month 3 to birth](https://durgical.com/en/embryology/fetal-period-month-3-to-birth/) - [Fetal growth and growth restriction](https://durgical.com/en/embryology/fetal-growth-and-growth-restriction/) - [Principles of teratology: critical periods, dose and genotype](https://durgical.com/en/embryology/principles-of-teratology-critical-periods-dose-and-genotype/) - [Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis](https://durgical.com/en/embryology/prenatal-diagnosis-ultrasound-serum-screening-cell-free-dna-cvs-amniocentesis/) ## Related conditions - [Prematurity and its complications](https://durgical.com/en/conditions/prematurity-and-its-complications/) - [Preterm labour and birth](https://durgical.com/en/conditions/preterm-labour-and-birth/) [← All of embryology](https://durgical.com/en/embryology/) 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](https://medlineplus.gov/ency/article/002367.htm) (MedlinePlus (U.S. National Library of Medicine), 2026); [Fetal development: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002398.htm) (MedlinePlus (U.S. National Library of Medicine), 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/gestational-age-vs-fertilization-age-how-pregnancy-is-dated/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/gestational-age-vs-fertilization-age-how-pregnancy-is-dated/