Naegele's Rule calculates the estimated due date (EDD) as: EDD = first day of last menstrual period (LMP) + 1 year − 3 months + 7 days, equivalent to 280 days (40 weeks) from LMP. It assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles, the due date is adjusted accordingly. Only ~5% of babies arrive on their exact EDD; 80% deliver within ±2 weeks.

Gestational age (GA) counts weeks and days from the first day of the last menstrual period (LMP), not from conception. At a typical 40-week delivery, the fetus is ~38 weeks post-conception. GA is confirmed by first-trimester ultrasound (crown-rump length, CRL), accurate to ±5–7 days and considered more reliable than LMP dating, especially if cycles are irregular.

First trimester (0–13 weeks): rapid cell division, organ and neural tube formation, heartbeat detectable at ~6 weeks. Second trimester (14–27 weeks): fetal growth accelerates, anatomy scan at 18–22 weeks, movement felt (quickening) at 16–20 weeks. Third trimester (28–40+ weeks): lung maturation (surfactant production by ~36 weeks), weight gain, and positioning for birth.

First-trimester crown-rump length (CRL) ultrasound (7–13 weeks) is the gold standard for dating, accurate to ±5–7 days. Second-trimester biometry (biparietal diameter, femur length) is accurate to ±10–14 days. Third-trimester ultrasound is the least accurate for dating (±3 weeks). ACOG recommends adjusting the EDD if CRL differs by >5 days in the first trimester or >10 days in the second.

ACOG classifications: Early term: 37 0/7–38 6/7 weeks. Full term: 39 0/7–40 6/7 weeks (optimal for infant outcomes). Late term: 41 0/7–41 6/7 weeks. Post-term: ≥42 0/7 weeks (increased risk of stillbirth; induction typically recommended). Premature (preterm): <37 weeks. Extreme prematurity: <28 weeks. Viability threshold: ~22–24 weeks with intensive NICU care.

Nausea and vomiting of pregnancy (NVP) affects 70–85% of pregnant women. Onset is typically at 6 weeks (after hCG begins rising rapidly), peaks at 8–10 weeks, and resolves for most by 12–14 weeks. About 10% experience symptoms beyond 20 weeks. Hyperemesis gravidarum (severe NVP causing dehydration/weight loss) affects ~0.3–3% and requires medical treatment.

Week 6: heartbeat detectable via transvaginal ultrasound. Week 10: neural tube closes; embryo becomes a fetus. Week 12: nuchal translucency scan (Down syndrome screening). Week 18–22: anatomy scan (detailed structural review). Week 24: legal viability threshold in most jurisdictions. Week 28: third trimester begins. Week 36–37: group B strep (GBS) screen. Week 39: full-term designation.

Gestational age (GA) is measured from LMP (the clinical standard). Fetal age (conceptional age) is measured from conception, approximately 2 weeks after LMP. A fetus described as "10 weeks gestational age" is approximately 8 weeks post-conception. Ultrasound and clinical references use gestational age exclusively to avoid confusion.

First trimester: cell-free DNA/NIPT (10+ weeks, screens for trisomies 21, 18, 13), nuchal translucency + serum (11–14 weeks). Second trimester: quad screen (15–20 weeks), anatomy ultrasound (18–22 weeks), glucose tolerance test (24–28 weeks). Third trimester: Group B Strep culture (35–37 weeks), non-stress test (NST) and biophysical profile (BPP) as indicated.

IVF due dates are calculated from the egg retrieval date: EDD = retrieval + 266 days (38 weeks from fertilization). For frozen embryo transfers (FET): 3-day transfer EDD = transfer + 263 days; 5-day blastocyst EDD = transfer + 261 days. IVF dating is more precise than LMP dating because conception timing is exactly known.

Seek immediate care for: vaginal bleeding in second or third trimester; sudden or severe abdominal pain; severe headache with visual disturbances or swelling (possible preeclampsia); decreased or absent fetal movement after 28 weeks; signs of preterm labor (regular contractions, pelvic pressure, fluid discharge before 37 weeks); or fever above 38°C (100.4°F).

A full-term pregnancy is calculated as 280 days, or 40 weeks, counted from the first day of the last menstrual period (LMP) — this is the same 280-day figure this calculator's due-date estimate is built on. Because ovulation and conception occur roughly 2 weeks into that count, the pregnancy itself lasts closer to 38 weeks from actual conception. Only about 5% of babies are born on the exact estimated due date; the ACOG-defined full-term window is 39 0/7–40 6/7 weeks.

Pregnancy is commonly described as "9 months," but the 280-day (40-week) count from LMP is a better approximation of 9 calendar months plus a couple of weeks — some clinical references instead describe it as 10 lunar months of 28 days each. There is no contradiction: both are just different ways of expressing the same 280-day/40-week span this calculator uses to compute your due date.

Pregnancy is tracked in gestational weeks from the LMP: the first trimester (weeks 0–13) covers organ and neural tube formation; the second trimester (weeks 14–27) brings the anatomy scan (18–22 weeks) and quickening (16–20 weeks); the third trimester (weeks 28–40+) covers lung maturation and final growth. See the "Key pregnancy milestones by week" answer above and this page's week-by-week trimester breakdown for the specific milestones at each stage.