Due date calculator

Naegele's rule is the most widely used method for estimating the due date. It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). The method assumes a regular 28-day cycle with ovulation on day 14. The result is approximate — only about 5% of babies are born on the exact calculated date.

LMP (Last Menstrual Period) is the date of the first day of the last period before conception. It is the starting point for calculating gestational age and estimated due date, because the exact moment of fertilisation is difficult to establish. Doctors use LMP as the standard reference point in obstetrics.

A normal pregnancy lasts from 37 to 42 weeks counted from LMP. The date calculated by Naegele's rule corresponds to week 40 (280 days). Birth before week 37 is considered premature; after week 42 the pregnancy is considered post-term.

The calculated date is an Estimated Due Date (EDD). Research shows that most births occur within ±2 weeks of the calculated date. More accurate results are provided by ultrasound — especially a scan performed between weeks 8 and 14 of pregnancy, which allows measurement of the Crown-Rump Length (CRL).

If you do not know your exact LMP date, a doctor or midwife will estimate gestational age using ultrasound. A first-trimester scan is the most accurate method for dating a pregnancy. You can also try to estimate the LMP date by knowing roughly when you conceived (ovulation occurs about 14 days after LMP).

Gestational age is counted from the first day of the last menstrual period (LMP), not from the date of fertilisation. This means that at the moment of conception (about 2 weeks after LMP) a woman is already in week 2 of pregnancy. Full term is at the end of week 40. Our calculator displays complete weeks of pregnancy using the same convention.

No — the calculator computes the due date only for singleton pregnancies using Naegele's rule. In twin or multiple pregnancies the due date is usually shorter (about 36–38 weeks) and should be determined individually with the attending physician.

The calculator subtracts today's date from the calculated due date (LMP + 280 days). The result is the number of days remaining until the due date. A negative value means the calculated due date has already passed. Remember that the result is indicative and does not replace medical advice.

The first visit to a doctor or midwife is recommended between weeks 6 and 10 of pregnancy. At the first visit the doctor will confirm the pregnancy, assess fetal age, order laboratory tests and plan the next ultrasound. Do not delay, especially if you have chronic conditions or previous pregnancies were complicated.

The due date is only an estimated date and cannot be adjusted freely. In medical cases — such as fetal distress, pre-eclampsia or post-term pregnancy — doctors may consider inducing labour or performing a caesarean section. All decisions about the timing of delivery should be made exclusively in cooperation with the attending obstetrician.