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Robin Elise Weiss, LCCE

How many weeks pregnant?

By January 18, 2011

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Pregnant Woman Holding Ultrasound of her Baby

I first heard of counting a pregnancy in weeks at my very first OB appointment. But no matter how many times I tried to add up the months of pregnancy, it didn't work. There were way too many weeks to cram into nine months. Despite the fact that I married a mathematician, math is not my favorite subject, though I quickly saw the benefit of calculating the weeks of pregnancy based on a single date, like the last menstrual period (LMP), to help make a universal time line for all women.

Haven't you seen women who give you the number of months they are and it simply doesn't seem to be the same as the last woman who said this? I had a lady in our childbirth class who said that her first baby was born prematurely at seven months. But later on she said he was due four weeks after he was born. By my calculations this wasn't seven months. Then there was another mom who I met early on in pregnancy, her due date was nearly a month after I was but told me that she was four months pregnant when I had calculated that I was five months.

So, I find months of pregnancy very confusing. It would be hard to say that a mom or baby really needed to be doing x, y, or z by a particular point if we didn't really have a way to ensure that the weeks were equally calculated. All that said, do you find calculating months to be effective? Do you prefer months to weeks or vice versa?


Photo iStockPhoto

July 15, 2009 at 9:56 am
(1) Erin says:

Weeks! Attempting to count months was more than I could handle. Especially when some books told me I was in the 5th month at 4 months — because I was in the 1st month at 3 weeks pregnant?? It was confusing. But i liked hitting milestones more often when turning __ weeks then having to wait for months.
I do remember though being asked how far along I was by my husband’s co-worker. I replied 26 weeks and he asked “how many months is that?” I could only shrug.

July 18, 2009 at 4:19 am
(2) jessica says:

Hi, I hope maybe I can clear things up a little bit!
there are several ways to count time in pregnancy: week by week for 40 weeks, month by month for ten four week months, or month by month using the traditional 9 months.
for the traditional 9 months, the 1st month begins 7 days after the 1st day of your last menstrual period. eg: your last period began march 5, your 1st month would begin march 12, your 2nd month would be april 12, and so on. your due date would be 9months later, on december 12.
when weeks are specified (as they usually are by doctors and midwives) we talk about the number of weeks since the 1st day of your last period. so if march 5 was was a sunday, 17 weeks into the pregnancy would be completed 17 sundays from this day, or on july 2nd. when counting 40 weeks to delivery, you would end up with a slightly earlier due date than when counting months, december 10 in this case.
so when you say you are, for example, 16 weeks pregnant, its true you are 4months, but in a TEN month pregnancy, of four week months.
so, i am 19 weeks pregnant. in ten four week months, iam 4 months, 3 weeks pregnant.
counting the traditional 9 months, i was 4 months on july 12th!
i hope this makes sense and tries to clear this issue up a bit, it really confused me too at first. a great pregnancy buy is “what to expect when your expecting” by Arlene Eisenberg, Heidi Murkoff and Sandee Hathaway.
Exellent read (and its where i got the above info from)

July 18, 2009 at 11:07 am
(3) pregnancy says:

Thanks, Jessica. I guess my point is that it’s harder for your practitioner to calculate specifics based on this and MOST women won’t go as far as to calculate specifically which month method they are using. This is why weeks works much better for ensuring a healthy mom and baby.

July 19, 2009 at 12:17 pm
(4) leanne says:

my first day of my lmp was 9th april. based on this my due date would be round about 13th janurary. but the sonographer said my due dat is 27th jan?????????? confused so am i in my 14th to 15 wk or am i 12 – 13 wk????????

July 20, 2009 at 10:34 am
(5) Debralee Taylor says:

Yeah, it’s confusing, I prefer weeks but then some people can’t understand that & want to know how many months. Just remember that every calendar month is more than four weeks. I think 13.5 weeks is technically the end of the first trimester. I always used how many weeks/months I had completed, while some boks & websites didn’t.

Leanne – I had the same thing, when I went for my first ultrasound they said I was only 6 weeks along instead of 8 like I had calculated based on my LMP, so they pushed my due date farther back. I figured I had just ovulated later in my cycle than “normal.” Supposedly the measurements taken that early in pregnancy are the most accurate because later in pregnancy the babies grow at different rates so it gets harder to calculate.

July 20, 2009 at 11:35 am
(6) Roo says:

To clarify… you are actually 2 weeks pregnant when you are ovulating (even though you are not “pregnant” at all) according to the LMP method that the majority of practitioners use… 4 weeks pregnant at the point at which you would have gotten your period. This kinda sucks, but is the most commonly used method… There is another method using “weeks gestation” which would put a woman just missing her period at 2 weeks gestation (but 4 weeks pregnant).

I never used “months” unless I was talking to someone that wasn’t pregnant/hadn’t had a child before. THEN, it was easier to say 7 1/2 months pregnant than to say 30 weeks… Of course, now, I am at the end and am 9 months (36 weeks) – and I have to explain to people that we are pregnant a FULL 9 months (with your due date being the first day of the 10th month).

For those of you that had a 2 week difference in due date from what you calculated from your LMP – the u/s sees the baby at the actual stage of gestation that they are – therefore, the due date is an “estimate” with a 2 week variance on either side. Hope that helps. :)

July 20, 2009 at 12:43 pm
(7) Nicole says:

You should keep track of how far along you are by weeks. This is more accurate. They add 40 weeks from the first day of your last menstrual cycle. But you are a month further along when you reach the date of that month that is your due date. Okay so… if your due Oct 12, you are a month further along every 12th.

July 23, 2009 at 1:59 pm
(8) Dee says:

I have never understood the halves. Like right now I am 30 weeks pregnant and so most people would say 7 1/2 months, but why is it not 8 months. Technically the 2 wks that makes it a half month is 2 wks into your 8th month. The math involved shows us the last week of the month. 8×4 is 32 so 33 would be the first week of your ninth…right? Thats how I do it and just trust that no one is following along as closely as I am so there is no dispute. If someone asks me right now how far along I am I simply say I am in my 8th month.

August 6, 2009 at 12:00 am
(9) Lavinia says:

Ok so can someone explain to me how you count the weeks? Im really confused. The first day of my last period was May 31st. which counting weeks I figured I was about 7 weeks. I went to the Dr. today and she said I am only 5 weeks. How is that? Im so confused.. someone help and explain to me how my dr. got only 5 weeks when I thought I was further along.

August 7, 2009 at 6:45 am
(10) pregnancy says:

There are many reasons that you could be less or more pregnant than you thought. The most obvious is that your dates are wrong, meaning you did not get pregnant when you thought you did. Another thing to keep in mind is how both you and your practitioner are calculating your dates. For example, if you are going by the first day or your last normal period and your practitioner is going by an early ultrasound. Or is your practitioner about by your last period and you are going by your date of ovulation? There are other reasons so it’s best to discuss the discrepancy with your doctor or midwife for the best answer. Here is a primer on how to count weeks:


January 29, 2010 at 11:07 am
(11) Erica says:

I went to the hosp Dec 9, 2009 they took a sonogram and told me that I wasent even 3 weeks.. I went for a sonogram January 25,2010 and they told me I was 12 weeks.. That would make me 9 weeks.. I dont understnad

April 11, 2010 at 5:46 pm
(12) stacey says:

my last period was the 18th of feb it says i am 7 weeks pregnant, but i ovulated on the 3rd of march i am so confused

January 18, 2011 at 12:02 pm
(13) Kathy says:

Thinking about what women must have done a hundred or so years ago, I bet they were even more general than months. I can imagine a woman saying she is due “in the springtime”, for example. I think that would take off a lot of stress, not fretting so much about details. If good prenatal care = good nutrition, clean water, and supportive community (which fundamentally is true), then you can do that through the whole pregnancy, regardless what week it is, right? Or what month. It’s only when we make pregnancy and birth medical events that all those other things – tests, appointments, etc. – begin to matter. Personally, all those are what stress me out. Also, since a healthy pregnancy can last anywhere from 38 to 44 weeks, give or take, then counting weeks might make you really impatient near then end! Wouldn’t it?

November 15, 2012 at 7:46 pm
(14) Paula says:

I am a bit confused as well, according to my lmp I am 8 weeks and 5 days, and yesterday I had my ultrasound and it said I was 8 weeks 2 days, shouldn’t it be 6 weeks, according to comments above, and it makes sense, the lmp calculator is 2 weeks over… What could this mean??? Twins??

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