Gestational Age

Embryology - 29 Sep 2026        Expand to Translate
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Timeline human development

Gestational Age (GA), or menstrual age, is a clinical term usually given in weeks and days to describe human development timed from the first day of the last menstrual period (LMP).

For gestational age in assisted reproductive technology pregnancy 2 weeks are added to the fertilisation date.

This clinical age therefore differs by approximately two weeks from the research materials timed from fertilization (fertilization age or conceptional age). These second terms are generally not used clinically.

Note - This current website preferentially uses the approximate fertilization age, except for some clinical presentations, and may also list the GA.

A template link from the GA acronym brings readers to this current page, use your browser back button to return to the original page.

Links: Timeline human development | Menstrual Cycle | Week 1 | Assisted Reproductive Technology | Birth

Term Pregnancy

American College of Obstetricians and Gynecologists Committee - opinion no 579: definition of term pregnancy.

"In the past, the period from 3 weeks before until 2 weeks after the estimated date of delivery was considered "term," with the expectation that neonatal outcomes from deliveries in this interval were uniform and good. Increasingly, however, research has shown that neonatal outcomes, especially respiratory morbidity, vary depending on the timing of delivery within this 5-week gestational age range."

They recommended that "term" be replaced with the more specific terminology related to gestational age:

• early term - 37 0/7 weeks of gestation through 38 6/7 weeks of gestation.
• full term - 39 0/7 weeks of gestation through 40 6/7 weeks of gestation.
• late term - 41 0/7 weeks of gestation through 41 6/7 weeks of gestation.
• postterm - 42 0/7 weeks of gestation and beyond.

Links: Birth

Estimating Due Date (EDD)

• Known LMP and normal 28 day cycle.
• Calculate EDD by adding 280 days.
• Cycle Variations
  • cycle greater than 28 days add 1 day for each day above 28
  • cycle less than 28 days subtract 1 day for each day below 28
• Ultrasound can alter EDD calculation.

Perinatal Period

American Academy of Pediatrics Committee on Fetus and Newborn 2004 recommendations

• Gestational age (completed weeks) - time elapsed between the first day of the last menstrual period and the day of delivery. If pregnancy was achieved using assisted reproductive technology, gestational age is calculated by adding 2 weeks to the conceptional age.
• Chronological age (days, weeks, months, or years) - time elapsed from birth.
• Postmenstrual age (weeks) - gestational age plus chronological age.
• Corrected age (weeks or months) - chronological age reduced by the number of weeks born before 40 weeks of gestation; the term should be used only for children up to 3 years of age who were born preterm.

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Cite this page: Hill, M.A. (2026, September 29) Embryology Gestational Age. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Gestational_Age

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© Dr Mark Hill 2026, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G