Enter the due date, and pick a date for the probability
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The number is based on UK National Health Service data of 640,000 births in 2005. It's far from perfect, and there are a few likely errors in the
whole calculation:
It allows for the fact that your baby has not been born yet by telling you the percentage of the remaining
births from today which happen on the selected day.
There are different ways to calculate the start date of the pregnancy. I can think of an obvious one, but
the standard basis is the first day of the Last Menstrual Period (LMP), about 2 weeks earlier. Midwives are directed
by the survey to use the LMP data, but many probably use the (more accurate) date from an ultrascound scan.
There are only weekly statistics available. I've interpolated daily figures on a sliding average basis -
there is a small error in this approach
Figures are only recorded up to the end of week 42
No allowance is yet made for multiple births (generally born around 3 weeks earlier) or, caesarean
births or mothers ethnicity.
Other factors such as mothers age etc. seem to have little impact on the stats
Also, babies tend to be unpredictable, as you may be about to find out.