Monday, December 4, 2006

Birth rates by Gestation Week

In the early months of the pregnancy, we asked a number of doctors a question that we thought was pretty straightforward: "What percent of deliveries occur before the due date, and what percent are quite early (e.g., more than 3 weeks early)?"  The answer was always vague or irrelevant: "Not many", "Oh, it is a low probability", "You should not worry about that."  We were not asking because of worry, we were asking for information.  What is the probability that we will be having an early birth?  After pressing, it became clear that the doctors just didn't know the actual distribution of deliveries by week.  Surely the answer is out there.  

A brief search turned up a database of births in Aberdeen, Scotland (  I retrived some of the data and calculated the percentages of deliveries by week.  The figure below plots 4021 births in the Aberdeen area by gestation week (note that 'singleton' means just one baby).  40 weeks is the normal term.

A full 73.5% of births occurred in the 39th, 40th, and 41st gestation weeks.  13.0% occurred in the 37th week or earlier. And, only 2.3% occurred in the 42nd week or later (into the 42nd week the placenta starts to break down and doctors will rarely allow a pregnancy to linger more than 10 days past due date).

These data do not account for possible differences by a variety of factors including - whether it is first child or not, regional differences in health and prenatal care, trends across decades, etc. Even so, it is illuminating and a useful chart for probability minded parents-to-be.

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