A pregnancy is said to be 40 weeks long. The '40 week' estimate was first developed by a German obstetrician called Naegele in the 1800's (making it known as 'Naegele's Rule'). He declared that a woman's pregnancy should last 10 lunar months (or 280 days) or about 9 calendar months. However, Naegele used the first day of the woman's last menstrual period as a definite 'starting point' to base his calculations on, meaning the conception of a baby typically occurred about 2 weeks later (or two weeks before the woman's next period was due). Therefore, even though the baby only comes into existence at conception, they are already referred to as being '3 weeks old' in 'gestational age'. This means a pregnancy is really only 38 weeks long (or 266 days).
Image 4-53 is a timeline graph illustrating how you can calculate your baby's due date from the first day of your last period, or their conception date.
If a woman has a 28 day menstrual cycle, then 'week 1' of her pregnancy starts with the first day of her last period. Some women have longer menstrual cycles (meaning their period has already come earlier than this), or shorter cycles (meaning the period may come a few days later). In some cases, a woman will not have a 'last period' because she has recently stopped hormonal contraception (such as the pill), or is breastfeeding or has recently experienced a miscarriage. (These are discussed in depth in variations for conception.) If you do have regular periods, the first day of your last normal period is used to calculate (or 'date') when your baby will be due (often referred to as 'LNMP' or 'Last Normal Menstrual Period'). You can read more in how to calculate your baby's due date .