"Dear Abby" and the Length of a Pregnancy

The "Dear Abby" reference that appears in this handout appears in Larsen, R. J., and Marx, M. L. (1981) An Introduction to Mathematics Statistics and its Applications , Englewood Cliffs, NJ: Prentice-Hall, p. 159.

The following letter appeared in the "Dear Abby" column in the Tennessean (Nashville) on January 20, 1973:

Dear Abby,

You wrote in your column that a woman is pregnant for 266 days. Who said so? I carried my baby for ten months and five days, and there is no doubt about it because I know the exact date my baby was conceived. My husband is in the Navy and it couldn't possibly have been conceived any other time because I saw him only once for an hour, and I didn't see him again until the day before the baby was born.

I don't drink or run around, and there is no way this baby isn't his, so please print a retraction about the 266-day carrying time because otherwise I am in a lot of trouble.

San Diego Reader

Assume that the standard deviation of pregnancies is 16 days.

Questions

  1. If the mean length of pregnancies is 266 days with a standard deviation of 16 days, how likely is it that a pregnancy would last at least ten months and five days?

  2. How long would a pregnancy have to last to fall in the longest 5% of all pregnancies? Find the z-score that corresponds to the longest 5% of scores in a standard normal distribution.

  3. According to the Web site www.nctpregnancyandbabycare.com (as of September 16, 2002), the "official" definition of a premature birth is a baby "born before 37 completed weeks of pregnancy." What is the probability that a baby born within a "normal" pregnancy would be born before 37 completed weeks?

  4. My son was born 18 days after his due date (and he was also born on the 150th anniversary of the founding of the American Statistical Association!).

    1. Find the z-score for a pregnancy that lasts 18 days beyond the due date.
    2. Find the probability that a normal pregnancy would last at least 18 days past the due date.

  5. Between what lower and upper limits do the middle 95% of all pregnancies last? Find the z-scores for the lower and upper limits you determine.

  6. Pretend that you are writing for the "Dear Abby" advice column, and write a response to "San Diego Reader."

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