Calculating the date
Factors that can alter the date
Once expectant parents have a 'positive' pregnancy test, their first question often is "When is my baby due?" Caregiver's will refer to the baby's due date as the 'estimated date for delivery' ('EDD') or 'estimated date for confinement' ('EDC'). However, it is precisely that, an
ESTIMATE! Even if your baby's 'due date' is accurately calculated from the definite day they were conceived, there is still only a 5% chance they will be born precisely on their due date!
Because every woman and her baby are different, we really don't know how long it takes each individual mother to 'grow her baby'. What we do know is that it is normal and healthy for babies to be born at any time from about 37 to 42 weeks of the pregnancy. However, even though babies born during this 5 week period are fully mature and ready to be born, they still may be 3 weeks earlier or two weeks later, than the estimated 'due date'. (Babies born less than 37 weeks are regarded as 'premature' and babies born later than 42 weeks are regarded as 'overdue'.) Most babies are born a week either side of the estimated due date, but are more likely to come the week after the due date, rather than the week before. This is discussed in depth in
post dates.
Calculating the date
Pregnancy due dates are said to be 40 weeks long. The '40 week' estimation was first developed by a German obstetrician called Naegele in the 1800's (hence it is called 'Naegele's Rule'). He declared that a woman's pregnancy should last 10 lunar months (or 280 days), being about 9 calendar months. However, because Naegele used the first day of the woman's last menstrual period as a definite 'starting point' to base his calculations on, and conception typically happens 2 weeks after this time (or two weeks before the woman's next period is due) a pregnancy is really only 38 weeks long (or 266 days), being about 8 ½ calendar months.