When I got to know that I’m pregnant the first question was “How to calculate due date of baby?” You can never be sure in things like that; you can only expect. Even if you have accurately calculated your baby’s ‘due date’ there is still only a 5% chance it will be born precisely on this date!

Because every woman and her baby are unique, no one knows how long it takes each separatly taken mother to grow her baby. It is normal and healthy for babies to be born at any time from about 37 to 42 weeks of the pregnancy. The babies born during this 5 weeks’ period are mature and ready to be born, but sometimes it happens that a baby born 3 weeks earlier or later than estimated ‘due date’. In the first case (born less than 37 weeks) babies are called ‘premature’, and babies born later than 42 weeks are called ‘overdue’. As I know, in most cases babies are born either one week earlier or a week later their due date. My girl was born 4 days later then her due date.

Pregnancy is said to be 40 weeks long. This term was first developed by a German obstetrician called Naegele in the 1800’s. He said that a woman’s pregnancy should normaly last 10 lunar months (or 280 days) which is close to 9 calendar months. But Naegele used the first day of the woman’s last menstrual period as a definite starting moment to base his calculations on, as conception usually happens 2 weeks after this a pregnancy is really only 38 weeks long (or 266 days), being about 8 ½ calendar months. That is why, two weeks after conception a woman is said to be ‘4 weeks pregnant’.
Naegele’s rool is based on a regular menstrual period which is 28 days, so if you have such you can calculate due date of baby using it.

9 months + 7 days after the first day of your last normal menstrual period (or 280 days after this day).

To calculate due date of baby faster and easier use Due Date Calculator