Last Menstrual Period

The most accurate way to calculate the length of pregnancy is by knowing the date of conception. Delivery would then be expected to occur 38 weeks (266 days) later. However, as most women are unaware of the date of conception, the first day of the last menstrual period (LMP) is used to calculate the expected date of delivery (EDD). This is done by applying Naegele’s formula to the LMP as follows:
1. Add 7 to the days
2. Subtract 3 from the months
3. Add 1 to the years.

This means that pregnancy is 40 weeks (280 days) long and assumes that conception occurs 2 weeks after the LMP.

The LMP is unreliable (and therefore Naegele’s formula cannot be used) if:

• The date of the LMP is not accurately known.
• The menstrual cycle is not 28 days long.
• The menstrual cycle is irregular.
• The woman has only stopped taking the combined oral contraceptive pill (‘the pill’) within the last 3 months.
• The woman has bled in early pregnancy.
• The woman is breast feeding or has been pregnant in the preceding 3–6 months.