May
28
2011

  Mittendorf's Due Date

Out of curiosity, I did a little research to find out more about my midwife's statement that most births are at 41 weeks and 1 day rather than at 40 weeks. I discovered that the 40-week standard was determined by a doctor in the early 1800s. Here's some info I found out about this from a paper I found online:

Naegele’s Rule: The standard definition for gestational term is 266 days from conception to the date of the baby’s birth. This is also defined as 280 days, or 40 weeks, from the first day of the mother’s last menstrual period, a definition which assumes that the mother ovulates on day 14 of a 28 day menstrual cycle. The formula used to calculate due date is:

(LMP + 7 days) – 3 months = Due Date

This definition is based on observations, first reported by Franz Naegele in 1812, who believed that pregnancy lasted ten lunar months from the last menstrual period. It was not based on empirical data.

A more recent formula, that is more relevant to today's women with current nutritional and health standards exists, so I don't know why more doctors (and midwives) aren't using it. Here's a bit about the newer formula:

Mittendorf’s Observations of Gestational Term. In the 1980’s, Mittendorf noticed that birth dates for women in his practice, primarily second-generation Irish-Americans, averaged seven days past their “due dates”. He reviewed his records, then went on to review records of 17,000 births, and determined the average healthy, white, private-care, primiparous woman averaged 288 days from LMP to birth: 8 days longer than Naegele’s rule. Mittendorf and other researchers have further determined several factors that affect gestational term, including ethnicity, parity, nutrition, substance use, mother’s age, and mother’s size. Based on Mittendorf’s data, a more appropriate formula might be:

(LMP – 3 months) + 15* Days = Due Date

* Add 10, rather than 15, if mother is non-white, or multiparous

In this case, "multiparous" means any baby after you've already had one (which was the primiparous birth). Using this new formula, my due date is really June 4th rather than June 1. But still, it's just the top part of the bell curve, so it's the most likely day but not the likeliest day by a huge margin, just a small margin.

Nevertheless, this new information has me thinking that maybe next Wednesday isn't going to be a done deal. Maybe I'll have the baby later in the week.

If you get the impression that I spend a lot of time thinking about when I'm going to have a baby, you are right. I literally lie in bed and ponder this. And since I'm lying in bed for a good many hours of the day in addition to all night, there is plenty of opportunity for me to ruminate. And wondering when exactly it is going to happen has lately been occupying my mind more than other stuff like what the baby will look like, what sort of personality he will have, etc., although I have plenty of time to wonder about all that, too.