Pregnancy Calculator

Naegele's Rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP) to estimate the due date. Crucially, pregnancy is counted from the first day of bleeding, not from ovulation or fertilization — so the first two weeks of any 'pregnancy' are technically before conception even occurred. This convention persists because LMP is a date most patients can identify accurately, whereas exact conception is rarely known. Naegele assumed a 28-day cycle with ovulation on day 14; the calculator adjusts the EDD by adding (cycle length − 28) days for longer cycles or subtracting for shorter ones. The 280-day average comes from large-population observation, not biology — actual gestation length varies by ±2 weeks for most healthy term pregnancies. ACOG, NHS, SOGC, and RANZCOG all use the LMP method as the default starting point, with ultrasound confirmation in the first trimester.

If conception or ovulation date is known precisely — for example through fertility tracking, basal body temperature charting, or ovulation predictor kits — the due date can be calculated as conception + 266 days (38 weeks) instead of LMP + 280 days. This is mathematically more direct, since it skips the 14-day pre-conception window that LMP-based dating includes. However, 'conception date' is rarely a single calendar day in practice: sperm survive in the reproductive tract for 3–5 days, so fertilization can occur up to a week after intercourse. For couples actively tracking ovulation, conception-based dating typically lands within 2–3 days of an early ultrasound estimate, making it useful when LMP is uncertain or when cycles are irregular but ovulation was confirmed.

Crown-rump length (CRL) measured by ultrasound between 8 and 13 weeks is the single most accurate dating method, with a margin of error of just 5–7 days. ACOG Committee Opinion 700 recommends using the ultrasound-derived EDD instead of the LMP-derived EDD whenever the two differ by more than 7 days during the first trimester (or by more than 10 days at 14–16 weeks, 14 days at 16–22 weeks). After 22 weeks, ultrasound dating becomes much less reliable because fetal growth varies enough that biometry no longer pins down gestational age within a useful range. NHS, SOGC, and RANZCOG follow similar redating thresholds. If a first-trimester scan changes the EDD, that new date is used for all subsequent clinical decisions including induction timing.

IVF dating is the most precise method available because the exact day of fertilization is documented. For a 3-day cleavage-stage embryo transfer, add 263 days (266 minus the 3 days the embryo had already developed in the lab). For a 5-day blastocyst transfer, add 261 days. Frozen embryo transfers (FETs) follow the same math — what matters is the embryo's developmental age at transfer, not when it was originally created or frozen. Because IVF eliminates the uncertainty of when ovulation and fertilization happened, IVF-derived due dates are often used as the gold-standard reference even over first-trimester ultrasound dating in clinical research.

Pregnancy is divided into three trimesters: first trimester runs weeks 1–12 (organ formation, highest miscarriage risk early on), second trimester runs weeks 13–26 (visible growth, anatomy scan around 18–22 weeks, first felt movements typically 16–22 weeks), and third trimester runs weeks 27–40 (rapid weight gain, lung maturation, full term at 39 weeks). Some clinicians use weeks 13–27 for the second trimester — the convention varies but the underlying biology is the same. Milestone markers most patients track: heartbeat detectable on transvaginal ultrasound around week 6, first-trimester screening at 11–13 weeks, anatomy scan at 18–22 weeks, viability threshold around 24 weeks, third trimester at 28 weeks, and full term at 39 weeks (per ACOG's revised term definitions adopted in 2013).

The popular phrase '9 months pregnant' is approximate. Forty weeks works out to roughly 9.2 calendar months, which is why pregnancy week-counters and month-counters drift apart as the pregnancy progresses. Some references use 'lunar months' of 28 days each — under that convention, 40 weeks equals exactly 10 lunar months, which is the source of the 'ten months' phrasing in some cultures. For clinical decisions, only weeks-and-days matter — months are a layperson approximation. The calculator displays gestational age in weeks and days because that's what every prenatal guideline (ACOG, NHS, SOGC, RANZCOG) is written in.

Gestational age is counted from LMP. Fetal age (also called embryonic age or conceptual age) is counted from fertilization, which is approximately 14 days later. So a fetus described as '10 weeks gestational age' is biologically about 8 weeks old. Clinical pregnancy guidelines, ultrasound reports, prenatal vitamin schedules, and ACOG/NHS/SOGC/RANZCOG documentation all use gestational age — the calculator follows this convention. Embryology textbooks and lab research more often use fetal age. The 2-week offset between the two systems is a frequent source of confusion when patients compare pregnancy apps that use different conventions.

Naegele's Rule assumes a 28-day cycle with ovulation on day 14. Real cycles range from 21 to 45 days for most people, and ovulation can shift by a week or more in either direction. The calculator adjusts the EDD using the formula EDD = LMP + 280 + (cycle length − 28). For a 32-day cycle, that means LMP + 284 days; for a 24-day cycle, LMP + 276 days. If cycles are highly irregular (variation more than 7 days between cycles), LMP-based dating becomes unreliable and an early ultrasound is the recommended standard for establishing the due date. Polycystic ovary syndrome (PCOS), perimenopause, and recent hormonal contraception can all produce cycle irregularity that warrants ultrasound confirmation.

Twin and higher-order multiple pregnancies use the same 280-day Naegele's Rule calculation as singletons — the EDD shown is still the standard 40-week date. However, expected delivery timing differs significantly: most twins deliver around 36–37 weeks, triplets around 32–33 weeks, and quadruplets earlier still. Monochorionic twins (sharing a placenta) often deliver earlier than dichorionic twins. ACOG recommends planned delivery at 38 weeks for uncomplicated dichorionic-diamniotic twins, 36–37 weeks for monochorionic-diamniotic, and 32–34 weeks for monochorionic-monoamniotic. The EDD is still useful as a reference point for monitoring schedules and milestone tracking — but the 'expected delivery week' for multiples is not the EDD.

ACOG redefined pregnancy term categories in 2013 to discourage non-medically-indicated delivery before 39 weeks: early term (37 0/7–38 6/7 weeks), full term (39 0/7–40 6/7), late term (41 0/7–41 6/7), and post-term (42 0/7+). Only about 5% of babies are born on the exact EDD. Most providers in the US, UK, Canada, and Australia recommend induction by 41–42 weeks because stillbirth and complication risks rise after that point. NHS in the UK typically offers induction by 41–42 weeks; ACOG recommends individualized decisions between 41 0/7 and 42 6/7 weeks; SOGC Canada recommends induction by 41 weeks 3 days. The exact threshold varies, but 'past due' does not mean immediate delivery — it triggers monitoring and shared decision-making.

Standard prenatal visit schedules differ across the four English-speaking markets. United States (ACOG): every 4 weeks until 28 weeks, every 2 weeks 28–36 weeks, weekly after 36 weeks — roughly 12–14 visits for an uncomplicated pregnancy. United Kingdom (NHS): 10 appointments for first pregnancy and 7 for subsequent, with the 12-week dating scan and 20-week anatomy scan as the two ultrasound milestones. Canada (SOGC): every 4–6 weeks until 28 weeks, every 2–3 weeks 28–36, weekly after 36 — typically 10–12 visits, varying by province. Australia (RANZCOG): GP and midwife shared-care models predominate, with roughly 10–14 visits, dating scan at 8–13 weeks and morphology scan at 18–22 weeks. All four systems schedule the dating scan in the first trimester and the anatomy scan around 20 weeks.

The most frequent errors when calculating EDD: using the last day of bleeding instead of the first day (LMP is always the first day of the period); confusing conception date with LMP date (they differ by ~14 days); forgetting the cycle-length adjustment for non-28-day cycles; trusting LMP without ultrasound confirmation when cycles are irregular; assuming pregnancy 'ends' on the EDD when it's actually a midpoint of a 5-week term window (37–42 weeks); and conflating gestational age with fetal age (always 2 weeks apart). For IVF, the most common mistake is not accounting for embryo age at transfer — a 5-day blastocyst transfer adds 261 days, not 266. When in doubt, an early ultrasound between 8 and 13 weeks resolves all of these uncertainties to within a 5–7 day margin.