silver (03/30/83)
My wife is due May 21 so naturally I'm interested in the subject. I found that the Encyclopedia Britannica Macropedia has a GREAT article on the subject of "Pregnancy". They gave some information I haven't seen elsewhere, including: --Average gestation in humans = 266 days from conception; this jives well with other sources. --Extremes considered to be 250 to 285 days, though full-term births from 221 to 355 (!) days accepted as occurring. --Chances of birth vs. due date: within 0 days: 5% (i.e., on the exact date) within 4 days: 25% (add 20% over 8 total days) within 7 days: 50% (add 25% over 6 total days) within 14 days: 95% (add 45% over 14 total days) The last bit of information I haven't seen anywhere else. However, it appears impossible! How could you add only 20% over 8 days, then 25% over only 6 more? This is not a smooth curve. Can anyone shed some light on this subject? (Reply to net or me as appropriate). Alan Silverstein ucbvax!hplabs!hpfcld!ajs
jwb (03/31/83)
Determination of the length of gestation has more than one problem. It is usually determined from the time of last regular menstrual period since the exact time of conception is seldom known. There is therefore a natural variability if the last regular menstrual period assumed is correct. If, however, the last regular menstrual period was really bleeding due to a threatened miscarriage (in other words the person conceived earlier than assummed), or if a person missed a period and then conceived (ie later than assummed), then this adds a compounding factor. This problem of dating the conception time is of course compounded in a woman with irregular periods anyway. Thus, determining the time of conception accurately is not a trivial problem. A physician (or midwife) used to taking care of pregnant patients uses physical findings and symptoms and perhaps ultrasonic determinations of size to try to figure out when the assumed dates are not realistic. Since menstrual cycles are rhythmic, I can see where picking the wrong one to date conception would make peaks and valleys in the probability distribution. Jack Buchanan UNC-CH