Free due date calculator based on Naegele's rule — the standard US OB/GYN method. Enter your last period date to find your estimated due date, current pregnancy week, trimester, and days until delivery.
Based on Naegele's rule — standard US medical formula
US obstetricians calculate your estimated due date (EDD) using Naegele's rule: add 280 days — exactly 40 weeks — to the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, or if an early ultrasound shows a different gestational age, your OB/GYN may adjust the due date accordingly. The first trimester ultrasound (between 8–12 weeks) is the most accurate way to confirm gestational age, with a margin of just ±5–7 days. The later an ultrasound is performed, the wider the margin of error — by the third trimester, dating by ultrasound alone can be off by 2–3 weeks.
Only about 5% of babies arrive on their exact due date. Most births in the US occur between 38 and 42 weeks of gestation — anything from 37 weeks onward is considered full term under current ACOG guidelines. The CDC reports that the average gestational age at birth for singleton pregnancies in the US is approximately 38.7 weeks. Premature birth (before 37 weeks) affects roughly 1 in 10 US births and is the leading cause of infant mortality and morbidity. Knowing your pregnancy week and trimester helps you stay on schedule with prenatal appointments, genetic screenings, and the anatomy ultrasound — all timed specifically by gestational age.
Major organ formation begins. Morning sickness peaks around weeks 8–10. Your first prenatal visit is typically scheduled at 8–10 weeks, including a dating ultrasound and blood panel. Chromosomal screening (NIPT) is offered from week 10 onward.
Often the most comfortable stretch — nausea eases and energy returns. The anatomy ultrasound happens at 18–20 weeks to check fetal development and — if you choose — reveal the sex. Baby movements (quickening) are typically felt between weeks 18–22.
Rapid baby growth — the average newborn gains about half a pound per week in the final month. Braxton Hicks contractions become common. ACOG recommends having your hospital bag packed by 36 weeks. Group B Strep testing occurs at 35–37 weeks.
Standard US prenatal care: monthly visits from weeks 8–28, every 2 weeks from weeks 28–36, then weekly until birth. At each visit your OB/GYN checks fundal height, fetal heart rate, blood pressure, and urine. The schedule is carefully timed to catch complications early.
Pregnancy is dated from the first day of the last menstrual period rather than from conception, a convention that predates any means of detecting conception and persists because the LMP is observable while conception is not. The practical consequence is that gestational age runs about two weeks ahead of embryonic age — at "six weeks pregnant" the embryo is roughly four weeks old. Trimesters divide the 40 weeks into three roughly equal periods, and the due date is 280 days from LMP. First-trimester ultrasound is more accurate than LMP dating and supersedes it when the two disagree materially.
Gestational age = days since LMP ÷ 7Due date = LMP + 280 daysConception ≈ LMP + 14 days (28-day cycle)Trimesters: 1st weeks 1–13 · 2nd 14–27 · 3rd 28–40where:
Assumptions: Estimates only. Ultrasound dating in the first trimester is the clinical standard where it conflicts with LMP. IVF pregnancies are dated from transfer and do not use this method.
Work out how far along a pregnancy is and which milestones lie ahead.
Result23 weeks 4 days — due 8 December 2026
Viability thresholds and screening windows are all quoted in gestational weeks from LMP, so using embryonic age instead shifts every milestone by a fortnight. The 24-week mark, a common threshold in clinical guidance, arrives in three days on this dating.