Pregnancy Due Date Calculator

How is a due date calculated?

Your estimated due date (EDD) is most commonly calculated using Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This calculator uses that same method. It assumes a regular 28-day cycle with ovulation around day 14. Because pregnancy is dated from the LMP rather than conception, you are considered “two weeks pregnant” at the moment of conception — which is why the count can feel surprising at first.

How accurate is your due date?

A due date is an estimate, not a fixed appointment. Only around 1 in 20 babies are born on their exact due date. Most arrive within a window of roughly two weeks before or after. Babies born between 37 and 42 weeks are considered term. Your healthcare provider may adjust your due date after an early ultrasound, which measures the baby directly and is often more accurate than LMP dating, especially if your cycles are irregular or you are unsure of your LMP.

Understanding the trimesters

Pregnancy is divided into three trimesters. The first trimester (weeks 1 to 12) is when the baby's major organs form and is often accompanied by symptoms like nausea and fatigue. The second trimester (weeks 13 to 26) is frequently the most comfortable, with the anomaly scan typically performed around week 20. The third trimester (weeks 27 to 40) is when the baby grows rapidly and prepares for birth. This calculator shows which trimester you are in and your key upcoming milestones.

The importance of antenatal care

Regular antenatal (prenatal) care is one of the most important things you can do for a healthy pregnancy. The World Health Organization recommends a minimum of eight antenatal contacts during pregnancy to monitor your health and your baby's development, screen for complications such as pre-eclampsia and gestational diabetes, and provide guidance on nutrition and wellbeing. Booking your first appointment early — ideally before 12 weeks — gives you and your baby the best start.

Common dating mistakes and limitations

1. Irregular cycles thrown into a 28-day formula. Naegele's Rule assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter or irregular, LMP-based dating becomes less accurate — an early ultrasound is usually more reliable in this situation.
2. Not being sure of your LMP date. If you can't recall the first day of your last period with confidence, any date calculated from it inherits that uncertainty. Say so at your first antenatal appointment — dating will usually be confirmed by ultrasound instead.
3. Confusing gestational age with fetal (embryonic) age. Gestational age, the number this and every clinical due-date calculation uses, is counted from your LMP. Fetal age, counted from actual conception, is about two weeks less. Both are valid, but mixing them up leads to confusion about your true stage of pregnancy.

IVF, known conception date, multiples and high-risk pregnancy

This calculator uses standard LMP-based dating, which doesn't fit every situation. If you conceived via IVF, your due date is typically calculated from the actual conception or embryo transfer date rather than LMP, since that date is precisely known — your fertility clinic will usually give you this date directly rather than you needing to estimate it here. If you know your exact conception date some other way, due date is generally estimated as 266 days from conception rather than 280 days from LMP. For twins or multiples, “full term” is defined earlier than for a single baby, and multiples are often born before 40 weeks — your care team will discuss a more specific expected timeline with you. For high-risk pregnancies — for example with pre-existing health conditions, previous pregnancy complications, or a multiple pregnancy — care is individualised and closely monitored, and a general calculator like this one is a starting reference point rather than a substitute for your care team's guidance.

Common myths about due dates

1. You will give birth on your due date. Only around 1 in 20 babies are born on their exact due date — most arrive within about two weeks either side, and that's entirely normal.
2. If you're sure of your LMP, an ultrasound dating won't change anything. Early ultrasound measurements are often used to confirm or adjust the due date even when LMP is known, particularly in the first trimester when fetal measurements are most predictable.
3. All twins are considered full term at 40 weeks. Multiple pregnancies have their own definition of term, generally earlier than for a single baby, and are frequently born before 40 weeks as a planned, healthy outcome.
4. Once set, your due date never changes. A due date can be adjusted, most commonly after an early dating ultrasound provides a more precise measurement than LMP-based calculation alone.