Calculate your pregnancy due date, current trimester, and key milestones based on your last menstrual period (LMP) or conception date. Uses Naegele's Rule — the same method used by OB-GYNs across the USA. Free, instant, no sign-up required.
Based on LMP or conception date — Naegele's Rule
A pregnancy due date is calculated using Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812 and still used by OB-GYNs, midwives, and hospitals across the United States today. The formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This accounts for the fact that ovulation and conception typically occur around day 14 of a 28-day cycle — meaning the first two weeks of your counted "pregnancy" technically precede actual conception. Only about 5% of babies are born exactly on their calculated due date, but roughly 80% are born within 2 weeks of it. Your healthcare provider may adjust your due date based on an early ultrasound measurement — the first trimester ultrasound (8–12 weeks) is the most accurate dating method.
Pregnancy is divided into three trimesters: First trimester (weeks 1–13) covers early fetal development — the most critical period for organ formation and when miscarriage risk is highest. Second trimester (weeks 14–27) is typically the most comfortable — morning sickness subsides and fetal movement becomes noticeable (quickening) around weeks 18–22. Third trimester (weeks 28–40+) is characterized by rapid fetal growth and preparation for birth. Full-term birth is defined as 39–40 weeks; early term is 37–38 weeks; preterm is before 37 weeks. The American College of Obstetricians and Gynecologists (ACOG) recommends elective delivery not before 39 weeks unless medically indicated.
The embryo implants around week 3–4. Morning sickness peaks around weeks 6–9. The heartbeat is detectable at 6 weeks by vaginal ultrasound. By week 12, all major organs are formed and miscarriage risk drops from ~15% to ~2%. First prenatal visit is typically scheduled at weeks 8–10.
The "golden trimester" — energy returns, nausea decreases. Anatomy scan ultrasound at 18–20 weeks checks for abnormalities and can reveal sex. Fetal movement (quickening) typically felt at 18–22 weeks for first pregnancies, 16–18 weeks for subsequent ones. Glucose screening at 24–28 weeks checks for gestational diabetes.
Rapid brain and lung development. Fetal kick counts monitored from week 28 — 10 movements in 2 hours is reassuring. Group B Strep screening at 35–37 weeks. Most hospitals recommend delivery between 39–41 weeks. At 42 weeks (post-term), induction is typically recommended.
Standard US prenatal visit schedule: Every 4 weeks through week 28. Every 2 weeks from weeks 28–36. Weekly from week 36 until delivery. Total: approximately 12–14 visits for uncomplicated singleton pregnancies. High-risk pregnancies have more frequent monitoring.
Naegele's rule, in use since the early nineteenth century, estimates a due date by adding 280 days — forty weeks — to the first day of the last menstrual period. The convention is worth understanding: pregnancy is dated from the LMP rather than from conception, which typically occurs about two weeks later, so "forty weeks pregnant" describes roughly thirty-eight weeks of actual gestation. The rule assumes a regular 28-day cycle with ovulation on day 14; for longer or shorter cycles the estimate shifts by the difference. Only about 4% of births occur on the estimated date, and first-trimester ultrasound dating is more accurate, which is why clinicians revise the date when scan and LMP disagree by more than a few days.
EDD = First day of LMP + 280 daysEquivalently: LMP − 3 months + 1 year + 7 daysCycle adjustment: EDD + (cycle length − 28) dayswhere:
Assumptions: An estimate, not an appointment. Term is anything from 37 to 42 weeks, and first-trimester ultrasound supersedes LMP dating where they conflict. IVF pregnancies are dated from transfer and do not use this rule.
Apply Naegele's rule, then correct it for a cycle longer than the assumed 28 days.
ResultEstimated due date 14 December 2026 (adjusted for a 32-day cycle)
The four-day cycle adjustment matters clinically, because decisions about post-term induction are made against the dated due date. Fewer than 1 in 20 babies arrive on the estimate itself; the majority arrive within the 37–42 week window either side of it.