January 22nd, 2013

planet

Gestation time based on previous pregnancy length

You knew this was coming... now that I'm pregnant with #2, I'm curious about second baby data. And it's timed well, because I've gotten some emails from interested people along the same lines, so I added a new chart to my pregnancy due date data analysis pages: the subsequent pregnancy date search.

What it does, is you type in when your first baby showed up, and it reports out who else in the database with a second kid had their first at the same time. And you can expand the range to include people who just went into labor *about* when you did.

Now... I already had a chart that showed the average date for first vs. second vs. third time moms, and interestingly enough, all the averages were about the same. In fact the more survey takers I get, the more the same they all are... everybody has their babies around 40 weeks. I'd always heard that first babies come a little later, but that's not what the survey is reporting.

So I found an interesting thing about second babies, and I'm not sure how to quantify it exactly, it's sort of just a trend I see from typing in different numbers.

The trend is that everybody just sort of 1) stays on their side of the 40W average but 2) drifts towards the middle. If you had your first baby late, you'll have your next baby late too... but not as late. If you had your first baby early, you'll have your first baby early too... but not as early.

In fact a lot of the numbers I found just cut the time-from-due-date exactly in half. The women who went into labor eight days late with their first had an average second baby who was four dates late. The women who were four days early with their first were two days early with their second.

Maybe some person who knows more about biology or even statistics can tell me why this is true. You would think that if the overall average is 40 weeks, then any group of women you pick would have their babies at 40 weeks. Or you'd think that if genetic factors design a woman's body to take X number of days, the average for a given group of women wouldn't drift much when you look at their first vs. second babies.

In fact if this is a real trend, it's the first thing I've found that can edge a woman significantly away from the 40 week average, and I have analyzed a LOT of factors... whether you're having a boy or girl, what season it is, how your due date was determined, your cycle length, I even looked at the phases of the moon (knowing full well I wouldn't find a correlation, I just did it for fun). I almost thought I had something with age... but not really. The average 40 year old might be two days later than an average 20 year old, but two days is hardly anything to write home about. Finding groups with average four or six days off, well maybe that's something. I don't have any questions about race in the survey, which is a factor I've read about, it's just that race is so hard to ask about, too many people are more than one so I don't know how I could analyze that on an internet survey that's going to be mostly made up of Americans.

Anyway, the obsession goes on, even though due dates are still kind of a crap shoot, the survey has a number of women who were a week late the first time, two weeks early the second time. So I shouldn't do tons of analysis. But it's fun, so I do.

Oh and on average, people who had their first baby at 39W 5D also had their second baby then. So for practical purposes, I'm going to assume this one's coming for me on May 6th: anniversary of the Hindenburg disaster. That'll be a fun birthday theme someday, won't it?