Determining the Date of Conception
Because the human egg is capable of fertilization for only 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception. However, ultrasound determination of the date of ovulation has the same imprecision as does the ultrasound estimate of the gestational age and, therefore, a precise date of conception cannot usually be determined as with in vitro fertilization.
In addition, although a woman is most likely to become pregnant if she has sex on the day of ovulation conception may also occur from live sperm still in her reproductive tract on the day of ovulation if she had sex for up to five days before ovulation.
Is there a more accurate way to calculate a “due date”?
Yes. One way is to shift Naegele’s rule. Adding 282 days to the first day of the last menstrual period, instead of 280, provides a more accurate estimate. But, if available, an estimate based on a first trimester ultrasound is the best option. Using ultrasound-based due dates, 92% of pregnancies delivered within the normal range of 37-42 weeks; using due dates, this percentage fell to 87%.
One day off. Two days off. Who really cares? Actually, this bias matters a lot. An estimate off by even a couple of days significantly raises the percentage of women regarded as post-term (past 42 weeks), who are then urged to have inductions.
Consider the results of a UK study of over 24,000 women. The researchers studied inductions for post-term (past 42 week) pregnancies. When evaluated by ultrasound dates alone, they found the majority, 71.5% percent, of these pregnancies were not actually post-term.
So when are pregnant women actually most likely to deliver?
In her book Expecting Better, Emily Oster used the 2006 annual U.S. data for singleton births, from the Centers for Disease Control, to try to answer this question. She noted that in 2006 the most common week to deliver in was the 39th; about 30% of births occurred then. The data from 2012 look similar to those from 2006, but hint: there’s a problem with her approach.
What’s the problem? These data are fine for figuring out when a pregnant woman is most likely to deliver, assuming she is agnostic about how she delivers. But they are not particularly useful for figuring out when spontaneous labor is most likely. This is because the CDC birth data includes all deliveries. The CDC combines spontaneous births with scheduled inductions and c-sections, and these are typically scheduled for the 39th week of pregnancy.
Are there really enough scheduled inductions and c-sections to affect the distribution of births for the whole U.S.? Absolutely. Since 1990, the rate of medical inductions has more than doubled. In 2012, more than 1 in 5 pregnant women were induced, compared to just under 1 in 10 in 1990. In 2012, 32.9% of women had c-sections, a 60% increase from 1996, and about half of these c-sections were scheduled.