Pregnancy Due Date Calculator
Calculate your estimated due date and key pregnancy milestones from your last menstrual period or conception date.
Quick Answer
A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's rule. Only about 4% of babies are born on their exact due date — most arrive within 2 weeks either side.
How Pregnancy Due Dates Are Calculated
The standard method for calculating an estimated due date (EDD) is Naegele's rule, developed in the early 19th century. The formula adds 280 days — exactly 40 weeks — to the first day of the last menstrual period (LMP). This assumes a standard 28-day menstrual cycle with ovulation occurring on day 14.
For cycles that differ from 28 days, the calculator adjusts the ovulation date accordingly. If your cycle is 35 days, ovulation typically occurs around day 21 rather than day 14, shifting your EDD later by 7 days. If your cycle is 21 days, your EDD moves earlier.
Why the LMP Method, Not Conception Date?
Most women know when their last period started but few know their exact conception date. The LMP method provides a consistent, easily dated starting point. Gestational age — the medical measure of pregnancy — is counted from LMP, which means a pregnancy is technically "2 weeks old" at the moment of conception.
If you know your conception or ovulation date (for example, from IVF, ovulation tracking, or a positive ovulation test), our calculator lets you input that directly. The calculator adds 14 days to estimate the LMP equivalent and then computes your EDD from that.
Understanding Your Due Date Result
Your EDD is an estimate — statistically, only 4% of babies arrive on their exact due date. About 80% of births occur within 10 days either side. "Full term" is defined as 37–42 weeks of gestation. Births before 37 weeks are preterm; after 42 weeks is considered post-term and is typically managed by your obstetric team with induction considerations.
Your first-trimester ultrasound (performed at 11–13 weeks) will either confirm your calculated EDD or revise it based on the baby's crown-rump length measurement, which is accurate to within ±5–7 days. If the ultrasound date differs from your calculated date by more than 7 days, your obstetric team may revise the EDD.
Key Milestones to Look For
Once you have your EDD, your calculator shows key milestone dates: the end of each trimester, the anatomy scan window (18–20 weeks), and your due date. Your midwife or obstetrician will add to this with scheduled appointments: booking appointment (before 10 weeks), combined screening (11–13 weeks), morphology scan (18–20 weeks), glucose challenge test (24–28 weeks), and group B strep swab (35–37 weeks) in standard antenatal care.
Important
This calculator provides estimates only. Book an antenatal appointment with your GP or midwife as soon as you confirm your pregnancy. Do not rely on a due date calculator for medical decisions.