My due date, by the numbers
Today is my due date.
But the hours are ticking down and the likelihood of a punctual baby is just about zero at this point.
I realized this might happen as soon as I found out I was pregnant. My mom had three of us. We were all late. Very late. I was three weeks late, my older sister was as well, and my little brother was induced after two weeks post date.
My husband likes to remind me that he was four weeks early, so maybe we will just cancel each other out and this baby will come right on time. But I am convinced that it has nothing to do with him and that I am doomed to follow in my mother’s footsteps and carry this baby around for another three weeks (or until the doctor induces me).
So what is a “due date” and does our genetic make up have anything to do with it?
An article that was published this summer in Human Reproduction entitled “Length of human pregnancy and contributors to its natural variation” by Jukic and colleagues highlighted the importance of not putting too much weight on set due dates. By analyzing 125 pregnancies and calculating gestation age based on ovulation date rather than the less reliable date of last menstrual period, the researchers found that gestation length can vary quite a bit. Over a 37 day span, to be more specific. That puts the due date range for normal gestation length at five weeks! The median gestation length was 38 weeks and 2 days (keep in mind that due dates calculated by the last menstrual period would tack on about 14 days – so this would be around 40 weeks for those with true 28 day cycles). Basically, it wouldn’t be crazy if this baby came in two weeks, rather than, say, tomorrow. UGH.
They also tried to figure out what exactly was leading to shortened or lengthened gestation time. Although they were quite limited with their sample size, from the data they were able to mine, they did find that events in the first two weeks affected timing of delivery. This related to the time it takes for the little ball of cells to implant itself into the uterine wall and also how fast the hormone progesterone increases in the circulation after conception.
Again, this study was quite limited with its sampling. Of the 125 pregnant women included, 95% were high school educated white women. There were very few smokers and the average age was 29. So, someone like me, a 32 year old, educated white non-smoker might be able to glean information from this study, but it is hard to predict what other factors could contribute to gestation length unless you fit this profile.
With that said, in this study, the mother’s own birth weight did show some correlation with the gestation length. Since this was not true of the mother’s adult weight, it can be inferred that there is something going on with inheritance from the mother – whether it be on the fetal side (e.g. growth rate) or maternal side (e.g. uterus size, pelvic size).
What about the father? Will the hubs’ jump-the-womb-ship-early genes cancel out my late bloomer genes?
A 2006 paper “Maternal and Paternal Influences on Length of Pregnancy” published in Obstetrics and Gynecology by Lie, Wilcox and Skjeorven considered the input from both parents. Here the researchers combed the Medical Birth Registry in Norway to gather birth data for two generations. From this massive data set, they found that several aspects of delivery date could be correlated with heritable traits from both the mother and the father. For fathers, a higher birth weight correlated with a decreased gestation length for their babies but this did not hold true for the mother’s birth weight (in contrast with the Jukic et al. study). The authors suggest that fathers may pass down their fast growing baby genes and thus influence the timing of birth. However, comparing parental birth weight with baby birth weight, both parents showed a positive correlation. In addition, the gestation length for both mothers and fathers correlated with the gestation length of their babies.
Although these two correlating factors, parental birth weight vs. offspring birth weight and parental gestation length vs. offspring gestation length, were evident for both parents, the effect was twice as strong for the mothers. Therefore, the mother’s genes appear to have twice the influence over birth weight and gestation length than the father’s. The authors predict this uneven influence from the maternal side suggests that the delivery decision is split between the baby’s input and the baby maker’s input. Here is their logic – the baby is half dad genes, half mom genes. If it was solely fetal input setting the date, that baby would follow exactly in the parents footsteps with no maternal leaning. But this is not the case, the maternal side is more predictive of the outcome (nearly two-fold), therefore, there must be some additional input from the maternal side. And similar to what was suggested by Jukic et al., this could have to do with the inner workings of the maternal body as it carries and grows a tiny human.
So, can we predict when this baby is going to make her debut?
Here is the basic baby prediction math –
1/3 = super early baby genes from the hubs (4 weeks early)
1/3 = super late baby genes come from me (3 weeks late)
1/3 = super late baby maker machine (again, 3 weeks late)
While this might seem like I should expect this baby to arrive a little more than a week past my due date, to complicate this math even more, the correlations between mother/father/baby gestation length and birth weight are not one to one. According to the Norwegian study, an extra week in parental time is not the equivalent of an extra week in baby time. Rather an extra week for mother’s gestation length = 1.22 days; for the fathers, one extra week = 0.58 days.
Revised baby prediction math – (keeping in mind that maternal input and baby input from mom’s genes are indistinguishable for the week:day ratio)
super early baby genes – 4 (weeks) x 0.58 (days) = -2.32 days
super late baby/baby maker genes 3 (weeks) x 1.22 (days) = 3.66 days
average = 0.67 days
Ok, so even if I give myself a bit more credit, this math suggests that the baby should arrive… tomorrow.
But, then again, these are averages, based on correlations, and I am not Norwegian.
And, really, this baby will probably just come out whenever she damn well feels like it.