December is the most popular time to make new people

Based on data of birth records from 1973 to 1999 in the United States, the 5 most popular dates to get pregnant are all in December:

  1. December 11
  2. December 4
  3. December 18
  4. December 12
  5. December 17

These days are calculated by subtracting the average length of pregnancy from the most common dates of birth. According to a compilation of numerous medical studies, the average pregnancy length is 280 days.

So, what are the least common days of conception? One might think we could apply the same procedure used above but using the least common birthdays.  However, doing so would result in wrong answers. The list of least common birthdays offers some clues for why we cannot estimate the least common days of conception using this method:

  1. February 29
  2. December 25
  3. January 1
  4. December 24
  5. January 2
  6. July 4
  7. December 26
  8. November 28
  9. November 27
  10. November 26

With the exception of leap day (which is the least common birthday for obvious reasons), the rest of the least common birthdays fall on or near holidays (namely New Year’s, Christmas, Independence Day and Thanksgiving). Why is it relatively uncommon to give birth on holidays? Since a significant portion of births are scheduled, mothers and doctors have the choice to avoid certain dates. For this reason, the number of babies born on December 25 is likely significantly affected by scheduling choices rather than just a function of conception date.

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