A math lesson from Clare

Clare recently wrote me this as a part of a larger discussion about when she'll stop barfing:

"Here's a little math for you -- pregnancy is really forty weeks, not nine months, so when you divide forty by three you get 13.33 weeks a trimester. If you divide nine months by three it's three months a trimester. But, technically a month is 4.28 weeks. Multiply that by nine and it only equals 38.57 weeks. Really a pregnancy is longer than people think . . .and I'm not really even sure what week I'm in."

I wonder if there's a calculus function with a derivative that predicts the baby's gestation and weight. They seem to be able to figure out everything else with calculus. Maybe the pregnancy function could involve the magical, natural number e.