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This has already been alluded to in discussing the pro- ^ gressive signs. Pregnancy is generally dated from the last " appearance of the menses. In this, however, physiologists have differed; probably from the fact that many women have been disappointed by this calculation: and this ques- tion cannot be settled as long as it is impossible to exactly tell when conception takes place.

The accepted theory is now, that an ovum descends into the womb immediately before or after every menstruation; that it remains there eight or ten days, exposed to fecunda- tion; that, after this, it loses its vitality, and passes off, after which the female is not liable to conceive until the next operation of the ovaries. This theory has a great deal that is plausible, but has been found untenable in so many instances, that it is not to be relied on. An ac- coucheur of great renown and experience has given a hun- dred and fifty cases, in each of which he had noted the precise date of the last appearance of the menses. These cases, which will be found below, show the impossibility of making an exact calculation of the time of delivery from that date. i6 21 46 28 18 II 5 252 to 259 days. 37th week 262 " 266 " 38th " 267 " 273 « 39th " 274 " 280 « 40th It 281 " 287 " 41st It 288 " 294 " 42d II 295 " 301 it 43d It 303 " 306 11 44th "

It can be well understood, that if a woman conceives just before her menses are due, and the menses become sup-