In the following, the x axis is in weeks, relative to the due date. The y axis is in percentage of babies delivered during that week. The mean is -0.5 weeks, and the standard deviation is 2.12 weeks. Note that this mean implies that babies tend to be delivered before, not after, the due date (which contradicts the common wisdom).

If babies delivered more than two weeks early are factored out, the mean will shifts to the right of the origin. (In other words, if you want to force the data to support the statement "most babies are born after the due date" you have to discount any baby born more than two weeks premature.)

There's not much of a tail on the right, probably because labor is usually induced if a baby is more than two weeks late.

distribution of delivery dates

The following graph shows the cumulative percentage of babies delivered, relative to due date. (It is based on the same data as the graph above.)

cumulative distribution of delivery dates

This data was taken from Pregnancy, Birth, and Family Planning, Alan Frank Guttmacher, Penguin Books, New York, NY, 1986. In tabular form, it is:
Week Days Chance
28 189-196 1:625
29 196-203 1:625
30 203-210 1:525
31 210-217 1:240
32 217-224 1:240
33 224-231 1:240
34 231-238 1:115
35 238-245 1:58
36 245-252 1:39
37 252-259 1:22
38 259-266 1:11
39 266-273 1:5
40 273-280 1:3½
41 280-287 1:5 2/3
42 287-294 1:12
43 294-301 1:34
44 301-308 1:74

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