Counting Pregnancy Weeks

My husband and I had this disucussion all the time! Medically, the doctors count weeks- you are supposed to be pregnant for 40. So... if you do the "math" that equals 9.5 "calendar months" or 10 4-week months. Either way, there are usually a few extra days in each month, so try not to worry about what "month" you are in, so much as which week.

Also- due dates are arbitrary based on when you are most likely to have ovulated and implanted! It's all just an educated guess, as my doctor reminded me.

Most pregnancies last an AVERAGE of 38-42 weeks. Since 40 is in the middle of that range, that's where they assign the estimated due date. This is also why some docs won't consider you late in order to induce labor until closer to (or past) 42 weeks.

Since the AVERAGE menstrual cycle is 28 days, and most women KNOW when their last period began, but not necessarily when they ovulate, it's easier to count weeks of pregnancy beginning from the 1st day of the last period. Like previously stated, calendar months are not even, so they count weeks since they are a specified length of time.

I've seen the number of weeks per trimesters divided up several different ways. Here's how my doc said to look at it to make it easier to understand...

They give you 2 extra weeks at the beginning (before ovulation) because most women KNOW when their last period began but not necessarily when they ovulated. And they give you 2 extra weeks at the end because average pregnancy length is 40 weeks (remember the range is 38-42 weeks) from the first day of the last period. This variation makes your pregnancy look like this:

Weeks 1-6 = 1st month (1-2 before ovulation, & 3-6 after ovulation)
Weeks 7-10 = 2nd month
Weeks 11-14 = 3rd month
Weeks 15-18 = 4th month
Weeks 19-22 = 5th month
Weeks 23-26 = 6th month
Weeks 27-30 = 7th month
Weeks 31-34 = 8th month
Weeks 35-40 = 9th month (Wk 40 = estimated due date)

This is how my doc explained it to me to make it easier to understand. I hope that helps!

I am 29 weeks with my 5th boy....I have often wondered that myself...But technically there is 4.33 weeks in a month...I get them online for the what to expect website...they calculate it on there and email to me weekly...Or try baby center...Congrats

you do get an extra week because not all months are 4 week months. there are 4 months a year that have 5 weeks in them. so even though you are pregnant for 9 calendar months, you are actually pregant 40 weeks (or 10 months). confusing i know.

No, it makes no sense at all. Congratulations on your coming baby.

I think the reason doctors use weeks is because they are a standard length and months aren't. Months vary from 28 to 31 days which average 4.33 weeks per month. Trying to determine which month you are in is pretty arbitrary and people do it different ways i.e. do you count back from your due date or forward from your approximate conception date (not your LMP because remember the first 2 weeks you aren't actually pregnant yet) or should you start counting at the LMP date. Do it each way and you come up with a slightly different answers. Which one is right? Weeks are easier.

As for the book, I think they just used their own formula and had to do something to make sure they got all the weeks in so they randomly assigned them to a certain month. :-)

Ok i am 10 weeks and 3 days pregnant. So if I want to know what is going on with baby this week, I would look at week 11, but this can really vary, if you have longer than 28 day cycles when you conceive, your gestational age, which is normally 2 weeks behind your due date (ex. my current gestational age is 8 weeks even though I am 10 weeks pregnant) can be a lot different. None of it has ever actually made any sense to me but that is the way it works.

Pregnancy is actually 9 lunar cycles, not 9 calendar months. It's 40 weeks which is about 9 months and 2 weeks. February is the only month with 28 days and exactly 4 weeks, right? So every other month is more than 28 days and therefor, each 4 weeks does not equal one whole month, you have to account for the extra 2-3 days each month so there are a couple months that have 5 weeks instead of 4. It's really all quite logical if you just look at a real calendar instead of pretending that 28 days/4 weeks equals one month. Plus, one year is 52 weeks, right? If you're pregnant for 40 weeks, that leave 12 weeks that you are not pregnant. If 12 weeks is 3 months, then 12 months minus 3 months leaves 9 months that you ARE pregnant.

Anyhow...I hate that book. Some of the information about where the baby is each week is helpful, but mostly the book is just a bunch of garbage and should have been titled "What to expect when you turn over all your care and decisions to an OB in a hospital setting" :P lol

I loved the website babycenter.com for counting the weeks. They show the development and what the baby looks like inside the womb. I liked knowing what was going on with my baby. I didn't like what to expect when your expecting. It tells you everything that can go wrong and made me worry.