Dna

[Ladielo]

How do doctors calculate your pregnancy? Do they add two weeks and 7 days or subtract 2 weeks and 7 days

[AnnieBrooke]

I don't know where you got the "and 7 days" part, but the simplest answer is, doctors add two weeks at the front end.

Back in grandma's day when there were no ovulation test kits, a woman would miss her period and go see the doctor to learn if she was pregnant. He would do an exam and tell her she was. She would ask "How far along am I?" but of course, the doc would have no way to know when conception was, since ovulation gives no obvious signal. So the medical world hit on using the date of the last period before the woman got pregnant, as a start point for the pregnancy time period. A period is a big, obvious signal, not a hidden event like ovulation. They figured, a woman has a period, a couple of weeks later she ovulates and that is when she gets pregnant. So let's say the pregnancy time period is 40 weeks long (even though from conception it is only 38 weeks long) so we can start the count on the first day of the last period, a date the woman often knows.

As ppowb notes, your doc saying you were 9 weeks 6 days gestational age meant you were about 7 weeks 6 days from conception at that time. The first two weeks  are added to reach back to the presumed first day of your last period, to calibrate with how pregnancy is counted by doctors.

All medical training, textbooks, machines, doctors, nurses, books like "What to Expect When You're Expecting," etc. begin the pregnancy count on the first day of the last period, even now that we are well past Grandma's time and have ovulation test kits and ultrasounds that can do a better job of judging when conception was. It would be good if doctor realized that a woman asking how many weeks along she was, might be asking when she conceived, so the doc could explain that the medical count doesn't begin at conception. But it often doesn't occur to doctors that saying "x weeks pregnant" could be confusing.

[ppowb]

The standard way is to count weeks from the first day of your last normal period. For a 28 day cycle the ovulation for this pregnancy occurred 14 days later and sex in the five days before ovulation could have fertilized the egg.

Simply put, if the test says six weeks pregnant it was from sex four weeks-ish ago.