Find your due date, current week, and pregnancy milestones
This calculator uses the standard Naegele's rule (LMP + 280 days) to estimate your due date. Conception date adds 266 days. IVF transfer adds 266 days for day-3 embryos or 264 days for day-5 embryos.
Only about 4% of babies are born on their exact due date — most arrive within two weeks either side. Your healthcare provider may adjust your due date based on ultrasound measurements.
This tool is for informational purposes only. Always consult your OB/GYN or midwife for medical guidance during pregnancy.
The standard method uses Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If you know your conception date, add 266 days. For IVF, the calculation depends on embryo age at transfer. Your doctor may adjust your due date based on an early ultrasound, which is considered more accurate.
Pregnancy is counted from the first day of your last menstrual period, not from conception. So when you're "4 weeks pregnant," the actual fertilisation occurred about 2 weeks ago. Enter your LMP date into our calculator and it will show your current week and trimester instantly.
The first trimester covers weeks 1–12, when major organ development occurs and miscarriage risk is highest. The second trimester (weeks 13–26) is often when symptoms ease, the bump becomes visible, and you may feel the baby move. The third trimester (weeks 27–40) involves rapid growth as the baby prepares for birth. Most babies are considered full term from 37 weeks.
Only about 4–5% of babies are born on their exact estimated due date. Most are born within two weeks either side. Around 80% of births occur between 38 and 42 weeks. A due date is an estimate, not a deadline — your healthcare provider will monitor you closely from 40 weeks onwards.
Key milestones include: heartbeat detectable (around week 6), first trimester screening (weeks 11–13), anatomy scan (weeks 18–20), glucose screening test (weeks 24–28), Group B strep test (weeks 35–37), and the baby being considered full term (week 37). Our calculator shows your upcoming milestones based on your due date.
The standard formula — Naegele's Rule — adds 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For longer cycles, the formula tends to underestimate true gestational age; for shorter cycles, it can overestimate. This is why first-trimester ultrasound dating (which measures the embryo directly) is considered more accurate than LMP dating when the two disagree.
IVF pregnancies allow exceptionally precise dating because the fertilisation or transfer date is known exactly. A Day 3 embryo transfer adds 266 days; a Day 5 (blastocyst) transfer adds 264 days.
First trimester (weeks 1–13) — the period of highest miscarriage risk. All major organ systems form. Folic acid supplementation before and during this trimester significantly reduces neural tube defect risk.
Second trimester (weeks 14–27) — often the most comfortable period. The anatomy scan (typically 18–21 weeks) checks for structural abnormalities.
Third trimester (weeks 28–40+) — rapid growth and lung maturation. Term is 37–42 weeks; induction is typically offered at 41–42 weeks.
Only about 4% of babies are born on their estimated due date. Most arrive within two weeks either side, and this variation is entirely normal.
Due date estimation follows the Naegele rule as used by the NHS pregnancy week-by-week guide.
Figures are estimates for guidance only. See about this site — how we source data and what these tools can and cannot do.
Pregnancy is conventionally measured in weeks from the first day of the last menstrual period (LMP), not from conception. This means that by the time a pregnancy test is positive (around week 4), you are already considered four weeks pregnant. The total duration of 40 weeks (280 days) is counted from LMP, even though conception typically occurs around day 14 of a 28-day cycle — making the actual gestational age from conception approximately 38 weeks.
This convention exists because the LMP date is easier to identify reliably than the conception date, which can only be estimated. Naegele's Rule — add 280 days (or 9 months and 7 days) to the first day of the LMP — has been the standard due date calculation since the 19th century and remains the basis for clinical dating worldwide.
A first-trimester ultrasound (typically performed at 11–14 weeks) measures the foetus's crown-rump length (CRL) and uses this to estimate gestational age with greater accuracy than LMP alone. If the ultrasound date differs from the LMP date by more than 5–7 days in the first trimester (or more than 7–10 days in the second trimester), most clinicians will adjust the estimated due date to match the ultrasound measurement.
This matters because cycles are not always 28 days and ovulation does not always occur on day 14. Women with longer cycles (e.g., 35 days) typically ovulate later, meaning their conception date is later and a due date based on LMP alone would be too early. Women with irregular cycles are particularly likely to have their due date revised after scanning.
For IVF pregnancies, due dates are calculated differently because the exact date of fertilisation is known. For a Day 3 embryo transfer, add 263 days (38 weeks minus 3 days) to the transfer date. For a Day 5 blastocyst transfer, add 261 days (38 weeks minus 5 days). Some clinicians add 266 days from retrieval date regardless of transfer day — methods vary, so confirm with your fertility clinic.
IVF pregnancies are dated from conception rather than LMP, but are still reported in standard gestational weeks to allow comparison with population norms for foetal development and growth.
Pregnancy is divided into three trimesters. The first trimester runs from weeks 1–12 and covers the embryonic period (weeks 1–8, when all major organs form) and early foetal period. The risk of miscarriage is highest in this phase — approximately 10–20% of known pregnancies end in miscarriage, most in the first trimester.
The second trimester (weeks 13–26) is typically the most comfortable period. Foetal movement (quickening) is usually felt between weeks 16 and 25. The anomaly scan, offered in the UK at 18–21 weeks, checks foetal anatomy in detail. Viability — the point at which premature birth can potentially be survived with intensive care — is generally considered to be around 24 weeks.
The third trimester (weeks 27–40+) involves rapid foetal growth and preparation for birth. Post-term pregnancies (beyond 42 weeks) carry increased risks and most clinicians will discuss induction. Only about 4% of babies are born on their estimated due date — the majority arrive in the two-week window either side.
Due date calculation follows Naegele's Rule as used in clinical obstetric practice. IVF dating methods follow standard fertility clinic protocols. Trimester definitions follow WHO conventions. Miscarriage statistics are from the Royal College of Obstetricians and Gynaecologists. This tool is for informational purposes only — always confirm your due date and pregnancy progress with your midwife or obstetrician.
Researched and maintained by Iulian, founder of Flux Media Systems. General information, not professional advice — about this site & our sources →