Pregnancy Calculators

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Pregnancy (gestation) is the period of fetal development from fertilization to birth, typically lasting 40 weeks (280 days) from the last menstrual period (LMP) or 38 weeks from conception. It is divided into three trimesters of approximately 13 weeks each. Key pregnancy hormone: human chorionic gonadotropin (hCG), produced by the trophoblast cells of the developing placenta, detectable in blood and urine 8–10 days after fertilization — the basis of pregnancy tests. Pregnancy causes major physiological adaptations: blood volume increases 40–50%, cardiac output increases 30–40%, and glomerular filtration rate (GFR) increases 50%.

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Trimesters

  • First trimester (weeks 1–13): Organogenesis; major fetal organs form; highest risk of teratogen damage; morning sickness; hCG peaks at 10–12 weeks; miscarriage risk highest
  • Second trimester (weeks 14–27): Fetal growth; movement felt (quickening ~20 weeks); anatomical ultrasound at 18–20 weeks; lowest miscarriage risk; sex determination by ultrasound
  • Third trimester (weeks 28–40): Rapid weight gain; lung maturation; fetal position; viability threshold at 22–24 weeks; preterm birth < 37 weeks

Due Date Calculation

Naegele's rule: EDD = LMP + 280 days (or LMP + 9 months + 7 days). First trimester ultrasound crown-rump length (CRL) is the most accurate dating method (±5–7 days at 8–12 weeks).

Key Physiological Changes

Blood volume: +40–50%. Cardiac output: +30–40%. GFR: +50% (lower creatinine normal). RBC mass: +20%. Respiratory: tidal volume +40%; RR unchanged; progesterone drives hyperventilation. Blood pressure: falls in first/second trimester (vasodilation); rises in third trimester.

Glossary

Gestational Age
Pregnancy duration measured from LMP (last menstrual period); full term = 40 weeks; preterm < 37 weeks; post-term ≥ 42 weeks; Naegele's rule: EDD = LMP + 9 months + 7 days.
hCG (Human Chorionic Gonadotropin)
Hormone produced by trophoblast after implantation; detectable 8–10 days post-fertilization; doubles every 48–72 h in early pregnancy; basis of pregnancy tests; peaks at 10–12 weeks.
Trimesters
First (weeks 1–13): organogenesis; hCG peak. Second (14–27): growth; anatomy scan; lowest miscarriage risk. Third (28–40): lung maturation; rapid weight gain; viability threshold at 22–24 weeks.

Frequently Asked Questions

Pregnancy duration is conventionally measured from the last menstrual period (LMP), not from conception (which occurs ~2 weeks after LMP). Full-term pregnancy: 40 weeks (280 days) from LMP = 38 weeks from conception. Naegele's rule for estimated due date (EDD): EDD = LMP + 280 days = LMP + 9 months + 7 days. Example: LMP January 1 → EDD: add 9 months (October 1) + 7 days = October 8. Ultrasound dating: crown-rump length (CRL) at 8–12 weeks is the most accurate dating method (±5–7 days). Second trimester ultrasound: less accurate (±2 weeks). If ultrasound disagrees with LMP by > 10 days (first trimester), adjust EDD to ultrasound.

Human chorionic gonadotropin (hCG) is a glycoprotein hormone produced by trophoblast cells of the developing placenta immediately after implantation. hCG prevents luteolysis (maintains corpus luteum → continued progesterone production → prevents shedding of the endometrium). hCG detection: urine or serum pregnancy tests detect β-hCG subunit. Blood hCG detectable at 8–10 days post-fertilization. Urine pregnancy test: positive when serum hCG > 10–25 IU/L. hCG levels: doubles every 48–72 hours in normal early pregnancy. Peaks at 10–12 weeks (50,000–100,000 IU/L), then declines. Ectopic pregnancy: hCG rises but less rapidly; no intrauterine gestational sac → high suspicion. Molar pregnancy: very high hCG.

Cardiovascular: blood volume increases 40–50% (plasma volume +50%; RBC mass +20%) → dilutional anemia is normal. Cardiac output increases 30–40% (HR +10–15 bpm; stroke volume +25%). Blood pressure decreases in first/second trimester (progesterone-mediated vasodilation); normalizes in third trimester. Renal: GFR increases 50% → serum creatinine, BUN, and uric acid fall normally in pregnancy. Creatinine > 0.9 mg/dL may indicate renal disease. Respiratory: diaphragm elevated by uterus; tidal volume +40%; minute ventilation increases; mild respiratory alkalosis (progesterone drives hyperventilation). pCO₂ normally ~30 mmHg in pregnancy. Metabolic: insulin resistance increases in third trimester (GDM risk); fasting glucose lower; postprandial glucose higher.

First trimester (weeks 1–13): Week 4: embryo implanted; cardiac activity begins week 5–6. Week 8: all major organs forming (organogenesis); embryo → fetus at week 10. Week 12: fetal kidneys producing urine; external genitalia differentiating. CRL ≈ 6 cm at 12 weeks. Second trimester (weeks 14–27): Week 16–20: mother feels fetal movement (quickening). Week 18–20: anatomy ultrasound. Week 22–24: viability threshold (50% survival with intensive care). Week 24–28: surfactant production begins (lung maturation). Third trimester (weeks 28–40): Week 28: 90% survival if born now. Week 32–34: accelerated brain growth. Week 36: lungs usually mature; considered late preterm. Week 37: early term; 39–40 weeks = full term. Week 41+: post-term; induction often offered.