Gestation Calculators

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Gestation is the period of fetal development from conception to birth. Gestation length varies enormously across species — from ~22 days in hamsters to ~22 months in elephants — and correlates with body size, brain complexity, litter size, and maturity at birth (altricial vs. precocial). In humans, normal gestation is approximately 40 weeks (280 days) from the last menstrual period (LMP), or 38 weeks from fertilization. Gestational age in clinical medicine is measured from LMP using ultrasound measurement of fetal dimensions. Preterm birth is defined as delivery before 37 completed weeks of gestation.

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Gestation Periods by Species

  • Hamster: 15–18 days
  • Mouse: 18–21 days
  • Rabbit: 28–35 days
  • Cat: 63–65 days
  • Dog: 58–63 days
  • Pig: 114 days (3 months, 3 weeks, 3 days)
  • Sheep: 145–155 days
  • Cow: 279–292 days
  • Human: 266 days (from fertilization) = 280 days from LMP
  • Horse: 330–345 days
  • Elephant: 660 days (~22 months)
  • Rhinoceros: 480–490 days

Why Gestation Length Varies

Larger body size generally = longer gestation (requires more development time). Brain size relative to body: humans have unusually long gestation relative to body size due to large brain development requirements. Altricial vs. precocial: altricial newborns (mice, dogs, humans) are born helpless → less maturation needed in utero but extended postnatal development. Precocial newborns (horses, cattle, deer) are born mature and mobile → longer intrauterine development required.

Human Gestational Age

Clinically measured from LMP: 40 weeks = full term (37–42 weeks = normal). Preterm: < 37 weeks (< 28 = extremely preterm; 28–32 = very preterm; 32–37 = moderate/late preterm). Post-term: > 42 weeks. Estimated due date: LMP + 280 days (Naegele's rule); confirmed by first-trimester ultrasound (crown-rump length measurement).

Glossary

Gestation
The period of fetal development from conception to birth; human gestation = 266 days from fertilization (280 days from LMP); measured clinically from LMP by Naegele's rule.
Preterm Birth
Delivery before 37 completed weeks of gestation; < 28 weeks = extremely preterm; 28–32 = very preterm; 32–37 = moderate/late preterm; leading cause of neonatal mortality in developed countries.
Altricial vs. Precocial
Altricial species (mice, humans) born helpless → shorter gestation + extended postnatal development. Precocial species (horses, deer) born mobile and mature → longer intrauterine gestation.

Frequently Asked Questions

Gestation is the period of fetal development from fertilization (conception) to birth. In humans: gestation from fertilization ≈ 266 days (~38 weeks). Clinical measurement is from LMP (last menstrual period) = 280 days (~40 weeks) because ovulation typically occurs 14 days after LMP and fertilization occurs around ovulation. Normal human birth: 37–42 weeks gestational age. Estimated due date = LMP + 280 days (Naegele's rule). First-trimester ultrasound (crown-rump length measurement, 6–13 weeks) is the most accurate method to confirm gestational age — more accurate than LMP dating, which assumes regular 28-day cycles.

Gestation length correlates with: body size (larger animals need more time to grow a full-size fetus — whale gestation ~16 months; mouse ~20 days). Brain development: species with larger brains relative to body size have longer gestations; humans have unusually long gestation for their body mass due to the demands of developing a large brain. Litter size: species with large litters (mice, pigs) have shorter individual gestations — multiple offspring, less development per pup at birth. Maturity at birth: precocial species (horses, cattle) born fully developed need long gestation; altricial species (mice, humans) born relatively helpless can be born sooner, with postnatal development continuing outside the womb.

Preterm birth is delivery before 37 completed weeks of gestation. Classification: Extremely preterm: < 28 weeks — very high risk; requires intensive NICU care; survival possible from ~22–23 weeks at specialized centers. Very preterm: 28–32 weeks — high risk; modern NICU support enables most to survive. Moderate to late preterm: 32–37 weeks — most survive with support; subtle long-term developmental risks. Causes: spontaneous (unexplained); cervical insufficiency; multiple gestation (twins/triplets); infections; uterine abnormalities; preeclampsia. Global: ~11% of births are preterm (~15 million/year); leading cause of neonatal mortality in the developed world. Prevention: cervical cerclage; progesterone supplementation; treatment of infections.

Gestational age is calculated from LMP: GA (weeks) = (current date − LMP date) / 7. Confirmed by ultrasound: first trimester — crown-rump length (CRL) ≈ gestational age in days - 42; accurate to ±5 days. Second trimester — biparietal diameter (BPD), femur length; accurate to ±2 weeks. Clinical uses: (1) Due date estimation — 40 weeks from LMP. (2) Fetal growth assessment — compare fetal measurements to gestational age norms (growth restriction or macrosomia). (3) Prematurity assessment — < 37 weeks = preterm; guide NICU readiness. (4) Prenatal screening timing — nuchal translucency (11–14 weeks); anatomy scan (18–20 weeks). (5) Lung maturity assessment — surfactant production adequate after ~34–36 weeks.