Reply #19: an answer, though it probably doesn't matter

[ant]

http://www2.hu-berlin.de/sexology/ECE2/html/beginning.html

A woman's pregnancy begins with the implantation of a blastocyst (i.e. the fertilized egg in a certain stage of development) in her uterus and, under normal conditions, ends about nine months later with the birth of a baby. However, for certain purposes of medical calculation the beginning of pregnancy is not dated from the time of implantation, but from the first day of the last menstruation. (The reason for this is that a woman is not aware of an implantation, but usually can recall the date of her last menstruation.)

I believe pregnancy time measured according to Last Menstrual Period (LMP) is usually noted as "X weeks LMP." I would also guess that these days doctors can better pinpoint the time of implantation based on fetal development.

In any case, the law actually doesn't kick in until "viability," which is up to the discretion of the individual doctor. If I remember correctly most abortions actually take place in the first 8-9 weeks of pregnancy. Since that's so early in the pregnancy, I would think the difference between time of last period and time of implantation is probably not enough to make a difference in terms of distinguishing a first trimester abortion from a second trimester one.

Edited to point out that since implantion will always take place after your last period, the error will be one that overestimates the pregnancy's progression. It seems the LMP calculation makes the pregnancy "older" than it really is.