how do they figure your due date?

Question

if i conceived 4 weeks ago, why do they say that i am 6 weeks pregnant. why do they count from your last period?

Answer 1

They count from the last day of your last period. This is also the reason they tell women that you can go into labor 2 weeks before or after your due date. If you know for a fact the exact day your child was concieved then that is great. Most women don't, so that is why the dr's time it the way that they do. It's no big deal.

Answer 2

The reason the 1st day of your last menstrual period is considered the first day of your pregnancy has to do with difficulties of estimating when conception actually took place. The biggest problem is that while most women ovulate around day 14 of their menstrual cycle, this can vary from month to month and from woman to woman, especially if a woman does not have regular cycles. There can also be some variability in the time it takes for fertilization to occur after ovulation, but this is a much smaller factor compared to the timing of ovulation itself.

Modern OBs rely heavily on ultrasound to help calculate a due date. This is most accurately done during the first trimester, and the rule of thumb is that an ultrasound during the 1st trimester is accurate within 1 week, 2nd trimester within 2 weeks, and 3rd trimester within 3 weeks. Ideally, you would like to have the date calculated from the last menstrual period to match well with the date calculated from the 1st trimester ultrasound. If ultrasound and last menstrual period give different dates, it's somewhat at the discretion of the OB to decide which should be used and if further tests might be needed.

You might also ask your doctor to see a "pregnancy wheel", which are fun gadgets most OBs use to do the math for calculating due dates.

Answer 3

You are actually pregnant 2 weeks before. You are actually pregnant 9 1/2 months. I don't know why though. They count from the last period because you got pregnant after that date.

Answer 4

Because usually when you have your period you are not pregnant. And they only count the first day of your period but you can get pregnant on your second day of period so thats an extra week. The pregnancy lasts up to 42 weeks. But if your baby is born over 38 weeks its classed full term. so thats 4 weeks leeway you have to count

Answer 5

The way that they figure your due date is to add 7 days to the first day of your last period the count back 3 months.

Answer 6

I dont know why they do that, but yes, you would be 6 weeks now. You are actually pregnant for 38 weeks which ends up being 9 1/2 months in weeks and with the 2 weeks they tack on at the beginning it ends up being 40 weeks (10 months in weeks). But because of the months not all being 4 weeks long, it does end up being 9 months in the end (in whole months).

Answer 7

Well you are pregnant 40 weeks,put that into months,its a bit more then 9 months. They count from your last period cause every womens body is different, so they don't know exactly when you ovulated, therefore just count from your last period. Hope that helps.

Answer 8

they count from your last period because the gestation period is actually 40 weeks. And because it is almost virtually impossible for most to determine the exact day they conceived they go by the date of your last period because it is for certain that you conceived sometie during that timeframe.... You ovulate for more than one day and the sperm can stay alive for up to 72 hours inside you so although you had sex on monday, egg fertilization may not have happened until 24 hours later.....

40 weeks to some is ten months but remember that some months have 5 weeks in them not just 4... which is why your pregancy is counted in terms of weeks by doctors....

Answer 9

I'm not sure, but to figure you due date, they start right from your last period and go 40 weeks. 40 weeks is actually 10 months, but that's the way they count it.

Answer 10

Day one of your pregnancy is day one of your period, believe it or not. That is the formula. Technically you aren't even pregnant until the 3rd week of your 'pregnancy' when you look at the formula. That is why it is so confusing and you are at 4 weeks when you think it has only been 2 weeks since you conceived.

So if you start your period on August 1st, then around August 15th you are 2 weeks pregnant and September 1st you are 4 weeks pregnant and so on.