Due date, gestational age, week-by-week baby development, medical milestone calendar, and visual countdown — everything in one place.
Pregnancy is measured in gestational weeks, counted from the first day of your last menstrual period (LMP) — not from conception. This is a longstanding medical convention that makes dating consistent even when the exact conception date is unknown. Because ovulation typically occurs around day 14 of a 28-day cycle, the pregnancy is already counted as approximately 2 weeks along at the moment of conception. A full-term pregnancy is therefore 40 weeks from LMP, though only 38 of those weeks represent actual fetal development.
The estimated due date (EDD) is calculated as LMP + 280 days (Naegele's Rule). Only about 5% of babies are born on their exact due date, but roughly 80% are born within 2 weeks on either side. A pregnancy is considered full term from week 39 through week 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond.
| Trimester | Weeks | Key developments | Key appointments |
|---|---|---|---|
| First | 1–13 | All major organs form; heartbeat begins at ~6 weeks; embryo becomes fetus at week 9; miscarriage risk highest and then drops sharply after week 12 | First OB visit (8–10 wks), NIPT (10–13 wks), NT scan (11–14 wks) |
| Second | 14–27 | Rapid growth; movement felt 16–22 weeks; hearing develops; sex often visible on ultrasound; viability milestone at 24 weeks | Anatomy scan (18–20 wks), quad screen (15–20 wks), glucose test (24–28 wks) |
| Third | 28–40 | Lung maturation; brain development; fat accumulation; baby moves into head-down position; rapid weight gain of ~0.5 lb/week from week 28 | Tdap vaccine (27–36 wks), GBS test (36–37 wks), weekly visits from 36 weeks |
The standard method used by all obstetric providers worldwide. Dating from LMP is reliable when cycles are regular (26–32 days) because ovulation is predictable. If your cycles are significantly longer or shorter than 28 days, your provider will typically adjust the EDD based on first-trimester ultrasound, which is more precise than LMP dating for cycles outside the normal range.
More precise than LMP dating for women who know their exact conception date through ovulation tracking, positive ovulation tests, IUI, or IVF. EDD = conception date + 266 days (38 weeks). For IVF pregnancies, the calculation uses the egg retrieval date (day 0) or the transfer date adjusted for the embryo's age at transfer (a 5-day blastocyst transfer subtracts 5 from 266, giving EDD = transfer + 261 days).
First-trimester ultrasound (before 14 weeks) is the most accurate dating method available, with a margin of error of ±5–7 days. The crown-rump length (CRL) measured between 6 and 14 weeks correlates precisely with gestational age. If ultrasound dating differs from LMP by more than 5–7 days in the first trimester, providers typically adjust the EDD to the ultrasound date. After 20 weeks, ultrasound dating becomes less precise (±2–3 weeks) and LMP dating is generally maintained unless there is a significant discrepancy.
A fetal heartbeat is typically detectable by transvaginal ultrasound at 6–7 weeks, beginning around 100–160 beats per minute. Seeing a heartbeat at 6 weeks reduces the miscarriage risk from approximately 20% at the time of a positive test to around 5%. A normal heartbeat range through the first trimester is 90–170 bpm.
Non-invasive prenatal testing (NIPT) screens cell-free fetal DNA in maternal blood for chromosomal conditions including trisomy 21 (Down syndrome), trisomy 18, and trisomy 13, as well as sex chromosome abnormalities. It can be performed from 10 weeks and has a detection rate over 99% for trisomy 21. The nuchal translucency (NT) scan measures fluid at the back of the baby's neck to screen for chromosomal differences and is performed between 11 weeks 2 days and 14 weeks.
The detailed anatomy ultrasound at 18–20 weeks evaluates all major organ systems, amniotic fluid levels, placenta placement, and fetal growth. It is the most comprehensive single prenatal screening. Most parents also learn the sex of the baby at this scan if they wish to. About 1 in 100 anatomy scans identifies a structural difference requiring follow-up imaging or specialist consultation.
Week 24 is recognized as the threshold of viability — the gestational age at which survival outside the womb becomes possible with intensive medical support. Survival rates at 24 weeks are approximately 50–70% with neonatal intensive care. By 28 weeks survival rates exceed 90%, and by 32 weeks exceed 98%. This milestone is why many parents feel a shift in emotional reassurance around the mid-second trimester.
A pregnancy is considered full term from 39 weeks (ACOG definition as of 2013). The previous standard of 37 weeks is now termed “early term” — babies born at 37–38 weeks have higher rates of respiratory issues, feeding difficulties, and NICU admission compared to those born at 39–40 weeks. Unless there is a medical indication, most providers aim to avoid elective delivery before 39 weeks.