Pregnancy Due Date Definitions

Naegele's Rule

In 1830, Franz Karl Naegele formulated a method for determining the Expected Date of Delivery. It was based on the first day of the woman's Last Menstruation Period (LMP). The calculations based on this rule follows. For demonstrative example we will assume January 1, 2007 as the First day of LMP. It is a three step process

Step 1: Add one year to LMP. Demonstrative Example: January 1, 2008

Step 2. Subtract three months. Demonstrative Example: October 1, 2007

Step 3. Add seven days to get the EDD. Demonstrative Example: October 8, 2007

This is fairly an accurate rule. We can fine tune this method.

Time of conception

Naegele's rule assumes conception taking place on the 14th day of a 28 day menstruation period. The menstruation period may be irregular, could be 24 to 45 days long. We can adjust this if we know the period.

Less than 28 days: Subtract number from EDD. 24 day period will make the Demonstration Example as October 4, 2007.

More than 28 days: Add number to the EDD. 32 day period will make the Demonstration Example as October 12, 2007.

Gestation Period

Naegele's rule of subtracting 3 months and adding seven days from the LMP date, introduces a a variability based on number of days in the the months being subtracted. An acceptable solution is to assume 280 days from LMP or 40 Weeks from LMP.

This 40 weeks of gestation includes 2 weeks to conception. So if we subtract 2 weeks or 14 days, it gives us 266 days or 38 weeks post conception for the delivery date.

Based on modern data, 268 days of conception weeks or 282 days for gestation weeks give more accurate results.

Further, you may add 10 more days if this is the first pregnancy.

Trimesters

Pregnancy period is broken down in to three periods. These periods are calculated differently by different physicians. The confusing part is that they are still known as 1st, 2nd, or 3rd trimesters, though the dates deepened on the assumption used.

Gestation Method

Divide gestation period of 280 days by three, it sets 93rd day from LMP as the last day of First trimester. 2/3 of 280 it sets ups 186th day from LMP as the last day of second trimester.

Conception Method

Divide 266 by three, giving you 89 as the last day of the 1st trimester starting from Conception day. 2/3 of 266 sets up 177th from conception as the last day of the second trimester.

Development Methods

This is the most common method. First trimester signifies completion of embryonic development . It happens at the end of 12 weeks from LMP (84 days). The fetus development reaches viability and starts to get ready for delivery at the end of 27 Weeks from LMP (189 days) denoted as the end of second trimester.

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