Baby's due date

‘When is my baby due?’ is one of the first questions many expectant parents will ask after a pregnancy test comes back positive. Most people know that pregnancies last about 9 months, but doctors and midwives measure the length of pregnancy in weeks. Your estimated due date can be calculated by counting ahead 40 weeks (or 280 days) from the first day of your last period.

When does pregnancy begin?

Pregnancy actually begins at the moment of conception — when the sperm and egg meet. This must happen within a day or 2 of ovulation — release of the egg from the ovary, which occurs midway through a 28-day menstrual cycle.

However, most women do not know the exact date of their baby’s conception. Because of this, the date of the first day of your last menstrual period (about 2 weeks before actual conception) is taken as the start date of pregnancy. So for the first couple of weeks of a 40-week pregnancy, the woman is not actually pregnant at all!

How long does pregnancy last?

While the estimated due date is calculated based on a 40-week pregnancy, the normal duration of pregnancy ranges from 37 weeks to 42 weeks, depending on the length of your menstrual cycle and the date of your last period. Only about 4 per cent of women actually give birth on their due date.

Ultrasounds

Ultrasound scanning enables doctors to measure accurately the age of the pregnancy, especially in the early stages. This is especially helpful for women who have irregular cycles and for women who are not sure of the date of their last period.

Last Reviewed: 5 August 2013
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References

1. MayoClinic.com. Fetal development: The first trimester (updated 4 Dec 2012). http://www.mayoclinic.com/health/prenatal-care/PR00112 (accessed Jul 2013).
2. MayoClinic.com. Prenatal care: 1st trimester visits (updated 4 Aug 2012). http://www.mayoclinic.com/health/prenatal-care/PR00008 (accessed Jul 2013).
3. FamilyDoctor.org. Ultrasound during pregnancy (updated Feb 2011). http://familydoctor.org/familydoctor/en/pregnancy-newborns/fetal-health/ultrasound-during-pregnancy.html (accessed Jul 2013).
4. US National Library of Mediicne, National Institutes of Health. MedlinePlus. When you pass your due date (updated 31 May 2012). http://www.nlm.nih.gov/medlineplus/ency/patientinstructions/000515.htm (accessed Jul 2013).
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