Due Date Calculator

How is the due date calculated?

Your estimated due date (EDD) is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method, known as Naegele's rule, is the standard used by healthcare providers worldwide.

Only about 4% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated date. Your healthcare provider may adjust your due date based on ultrasound measurements.

Pregnancy trimesters

• First trimester: Weeks 1–13
• Second trimester: Weeks 14–26
• Third trimester: Weeks 27–40+

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What is a due date, and how accurate is it?

Your estimated due date (EDD) is the date that is 40 weeks — or 280 days — from the first day of your last menstrual period. This calculation, called Naegele's Rule, has been used by obstetricians for over 200 years and remains the global standard today. That said, it is an estimate, not a guarantee: only around 4% of babies arrive exactly on their due date. The vast majority are born within a two-week window either side of that date, and anything between 37 and 42 weeks is generally considered a full-term, healthy delivery.

Why 40 weeks if conception happens at week 2?

Pregnancy is counted from the first day of the last menstrual period rather than from the moment of conception. This is because ovulation and fertilisation typically occur around day 14 of a 28-day cycle, but pinpointing the exact date of conception is rarely possible. Counting from the LMP gives a consistent, measurable starting point that everyone — patients and providers — can agree on.

What is Naegele's Rule?

Naegele's Rule adds 280 days to your LMP, or equivalently adds 7 days then adds 9 calendar months. For example, if your last period started on 1 January, your estimated due date would be 8 October. Our calculator applies this formula automatically, adjusting for your cycle length if it differs from the average 28 days. Women with longer cycles tend to ovulate later, so their due date shifts forward; shorter cycles shift it back.

Can my due date change?

Yes, and this is common. Your healthcare provider may revise your due date after a dating ultrasound, usually performed between 8 and 14 weeks. The ultrasound measures the size of the embryo (crown-rump length), which is currently the most accurate way to date a pregnancy, particularly if your periods are irregular or you are unsure of your LMP. If the ultrasound date differs from the LMP date by more than a week or two, most providers will adjust the EDD accordingly.

First trimester, second trimester, third trimester

Pregnancy is divided into three trimesters. The first trimester covers weeks 1–13 and is the period of the most rapid early development — by the end of week 10 all major organs are formed. The second trimester (weeks 14–26) is typically the most comfortable; the risk of miscarriage drops significantly and most people find their energy returns. The third trimester (weeks 27–40+) is the final stretch, during which the baby gains most of its weight and prepares for birth.

What happens at each prenatal visit?

Once your due date is established, your healthcare team will schedule regular check-ups. In a standard low-risk pregnancy, these happen roughly monthly until 28 weeks, fortnightly from 28 to 36 weeks, and weekly from 36 weeks onwards. Each visit typically involves checking blood pressure, measuring fundal height, listening to the baby's heartbeat, and reviewing any symptoms. At around 20 weeks, a detailed anatomy scan checks all the baby's organs and structures.