Find out your due date with our pregnancy calculator, by entering the date of the first day of your LMP (last menstrual period) and adjusting your cycle length. Your due and conception dates will be calculated automatically.
So how do you know your baby due date? First of all, the exact term is “estimated date of delivery”, which means you can deliver before or after the date. This date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP) (assuming you have a regular, average 28 day cycle). The full formula is a bit more complex and includes taking the first day of your last menstrual period, adding one year, subtracting three months, and adding seven days. The result is approximately 280 days from the start of the (LMP).
This was the standard way to calculate the due date It is known as Naegele’s rule and is still used today. Interestingly enough, it was devised in the first half of the 19th century, by the German obstetrician, Franz Karl Naegele. Another formula, suggested by Rakesh M. Parikh only a few years ago, takes into account cycle duration, which makes sense – after all, not all of us have an average 28 days cycle or even a regular one. Parikh suggests adding 9 months to the start of the last menstrual period, subtracting 21 days and adding duration of previous cycles to minimize errors in calculating the due date. The modern baby due date calculators are based on Parikh’s formula.
Since this is only an estimate (though decidedly a pretty accurate one), a normal pregnancy usually lasts anywhere from 38 to 42 weeks, and most babies are born within two weeks either way of the estimated due date. Of course, if your cycle is irregular and you’re not sure when you last ovulated and conceived, it might be trickier to determine how far along you are. In addition, your doctor may change your EDD as the pregnancy progresses for a number of reasons, including an irregular cycle.