Due Date Calculator

A due date calculator estimates your baby's birth date based on your last menstrual period (LMP). The standard medical method uses Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last period. The first 14 days are the follicular phase before ovulation, and conception typically occurs around day 14 after your LMP. The due date is an estimate - only about 5% of babies are born on their exact due date, and most arrive between 37 and 42 weeks, which is considered normal. For medical, legal, or financial decisions, the result should be treated as guidance and checked against official rules.

The classic obstetric estimator is Naegele's rule, formalized in the early 19th century: estimated due date (EDD) = LMP + 280 days = LMP + 9 calendar months + 7 days. The rule assumes ovulation on cycle day 14 of a regular 28-day menstrual cycle, so the 280-day total counts from the first day of the last menstrual period (LMP) rather than from conception. Conception age (post-fertilization age) is therefore gestational age minus 14 days, which is why fetal-development charts and LMP-based clinical charts disagree by about two weeks for the same pregnancy. Implementation in JavaScript uses Date arithmetic that ignores DST and leap-year boundaries because the calculation is in whole days: new Date(lmp.getTime() + 280 * 24 * 60 * 60 * 1000) is exact only when ignoring timezone offsets, which is why a UTC anchor (Date.UTC(y, m, d)) is the safer choice. Naegele's rule is a population estimate, not a personal prediction. ACOG Committee Opinion 700 states that first-trimester ultrasound measurement of the embryo's crown-rump length (CRL) is more accurate than LMP dating when LMP is uncertain or the cycle is irregular: at CRL 7-60 mm (gestational age 6 weeks 0 days to 13 weeks 6 days), ultrasound dating is accurate to ±5-7 days, and the EDD should be revised when the ultrasound and LMP estimates disagree by more than the published thresholds (e.g. > 5 days in the first trimester). For longer or shorter cycles, the EDD can be adjusted by (actual cycle length - 28) days. IVF pregnancies use embryo transfer date plus 266 days (day-5 blastocyst transfer) or 268 days (day-3 transfer) for higher precision. Normal labour occurs between 37 weeks 0 days (early term begins) and 41 weeks 6 days; under ACOG terminology 39+0 to 40+6 is full term, and only about 4-5 percent of pregnancies deliver on the EDD itself. The pregnancy is divided into three trimesters (1st: 0-13+6 weeks, 2nd: 14+0-27+6, 3rd: 28+0 to delivery). For twin and higher-order multiples, scheduled delivery is typically earlier (e.g. 38+0 for uncomplicated dichorionic-diamniotic twins per ACOG Committee Opinion 764) and Naegele's rule alone is not used.

• Naegele's rule: EDD = LMP + 280 days = LMP + 9 calendar months + 7 days; assumes day-14 ovulation in a 28-day cycle.
• Conception/fertilization age = gestational age - 14 days; fetal-development charts based on conception age are offset two weeks from clinical LMP charts.
• Cycle adjustment: when actual cycle length ≠ 28 days, shift EDD by (cycle_length - 28) days; e.g. a 35-day cycle pushes EDD 7 days later.
• First-trimester crown-rump length (CRL) dating per ACOG CO 700 supersedes LMP dating when the LMP-vs-ultrasound discrepancy exceeds the trimester-specific threshold (> 5 days at < 9 weeks, > 7 days at 9-15+6 weeks).
• IVF dating: blastocyst (day-5) transfer EDD = transfer date + 261 days; day-3 embryo transfer EDD = transfer + 263 days; these supersede LMP entirely.
• ACOG term definitions: early term 37+0-38+6, full term 39+0-40+6, late term 41+0-41+6, post-term ≥ 42+0; only ~4-5% of pregnancies deliver on the exact EDD.
• Use Date.UTC(y, m, d) and integer day arithmetic to avoid DST and timezone-offset drift around midnight; adding 280 * 86400000 ms across a DST boundary in local time can shift the result by one hour and flip the date.