How To Calculate Your Pregnancy Due Date

Finding out that you’re pregnant is a very happy time. There are so many things to do like buying a crib, diapers, stroller, and much, much more!

Nine months seems like an eternity when you first find out you’re pregnant, but it really is not. There are so many milestones that happen and so much to do it will go by in a flash.

So you’re pregnant, what is a due date and when is it?

Your due date is the date that is given to you of when you can expect to go into labor. The dates are not usually very accurate. More often women go into labor with 2 weeks of the expected due date.

The most accurate way to determine a due date is when the woman knows the day of conception. This rarely happens so normally due dates are determined by using the 1st day of the woman’s last period. Normally the total gestation time is added to the 1st day of the last period minus 2 weeks.


The total gestation time is 40 weeks (9 months). If the woman’s last period was on October 1st, 2005, you would add for weeks to that date and the date you would have then would be July 1st, 2006. You take away 2 weeks from this date and you have June 15th, 2006. So your due date would be June 15th.

This method is called the LMP Method (last menstrual period).

Two weeks are taken away from the 40 weeks because it is said that women can ovulate within 2 weeks of their last period.

Another interesting way to calculate your due date is called Naegle’s Rule. As with LMP, Naegle’s Rule also uses the first day of your last period.

Calculate the 1st day of your last period.
Count back 3 months from that day
Add 7 days

This is another easy way to calculate your due date.

Ultrasounds can also give an expecting mother her due date. Ultrasound technicians use the ultrasounds to determine the baby’s age by looking at different body parts and organs.

There are also many online websites for expectant mothers that offer pregnancy due date calculators. These calculators often use the LMP method by asking you to put in the first day of your last period and calculating your due date from there. The calculators not only give the mothers a due date to anticipate, but can also be used to mark the different milestones of pregnancy.

For example:

the time when a baby develops organs
2nd trimester
3rd trimester
and your due date

In ancient times women used the different phases of the moon. When a woman suspected that she could be pregnant she would look at the moon and what phase it was in. Over time she would count how many times the moon was in that phase. At the 10th time the moon entered into that phase and the 10th period she missed, the woman would know she was due soon.

Dr. Naegle (with Naegle’s Rule) determined that the ancient way was also an accurate way to determine a due date since a woman’s cycle seems to follow by the moon’s cycle.

Although the ancient way is no longer used it normally came pretty close. Whether you want to calculate the moon’s phases or use LMP to calculate it, there are many, many ways to find out a round about date as to when your new baby will be born!


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