Months Vs Weeks

10 Replies
Steph - April 11

Found this info pretty helpful....and amusing! GO BY WEEKS!!!!! :o) ********Obstetrical care providers always use "weeks" when they talk about estimated gestation, or how far along you are in your pregnancy. This presents a problem for many pregnant women, because the majority of pregnant women think about how far along a pregnancy is in "months." There are good reasons why care providers use weeks instead of months for estimated gestation. A pregnancy is a rapidly moving event and a lot happens in a short time. A month is too long of a time period to be of any practical use when you're making medical decisions about a pregnancy. Another problem is that there is no agreement on what it means to be "six months" pregnant. Some people say that they are six months pregnant when they are in their sixth month of pregnancy, while others say that they are six months when they have completed their sixth month of pregnancy. This isn't a problem if you use weeks, because, at least in the United States, it is universally accepted that weeks gestation always refers to completed weeks of gestation. There is an even bigger problem with months—how long is a month? A week is simple, because all weeks are exactly seven days long. But if you're talking about months, February is either 28 or 29 days long, depending on whether it's a leap year. Other months are 30 days long and others are 31 days long. If months aren't the same length, how do you convert weeks gestation into months gestation? Most people would answer that a month is 4 weeks long, so you convert weeks gestation into months gestation by simply dividing the weeks by 4. By this logic, if you are 20 weeks gestation, then you must be 20 / 4 = 5 months pregnant. This seems logical until you try to calculate how many months you are when you are at full term, i.e., when you are 40 weeks gestation. If you divide 40 weeks by 4 weeks per month, you get 40 / 4 = 10 months!! I know of no pregnant woman who will agree that a pregnancy lasts 10 months! The reason why you get such a goofy result is that your calculation is based on a false mathematical premise, i.e., that there are 4 weeks in a month. Four weeks is only 4 x 7 = 28 days. Most months are 2-3 days longer than 28 days, and this 2-3 day difference per month introduces a systematic error into your calculation. So, if a month isn't 4 weeks long, just how long is a month? This is where it gets really complicated. We already know that months are not the same length, so we're going to have to compromise and use the average length of a month. If there are 365.25 days in a year and 12 months in a year, then the average length of a month is 365.25 / 12 = 30.4375 days. If you then divide the average length of a month by seven days per week, you can see that an "average" month is 30.4375 / 7 = 4.3482 weeks long which rounds to 4.35 weeks per month. So if you divide the weeks gestation by 4.35, you should get months gestation. This almost works. If you divide 40 weeks gestation by 4.35, you get 40 / 4.35 = 9.1954 months which rounds to 9.2 months!! At this point you may be thinking: "What do you mean 9.2 months?" The reason why you get 9.2 months when you convert 40 weeks gestation to months using a conversion factor of 4.35 weeks per month is that 40 weeks is actually about 6 days longer than 9 months. The fact that a pregnancy is six days longer than nine months is implicitly acknowledged in Naegele's Rule which states that the due date is calculated by subtracting three months from the first day of the last menstrual period and adding one week. Even though it can be mathematically proven that a pregnancy is 6 days longer than 9 months, most people find this calculation unsatisfying. Ok! Let's try this one more time! This time we're going to divide 40 weeks by 9 months to get 40 / 9 = 4.4444 weeks per month which rounds to 4.44 weeks per month. If you divide 40 weeks gestation by 4.44 weeks per month, you get 40 / 4.44 = 9.009 months which rounds to 9.0 months.! Ok! Now that's more like it! The problem with using 4.44 weeks per month as a conversion factor is that it's rather artificial. If you assume that months are an average of 4.44 weeks long, then you have to also assume that months are 4.44 x 7 = 31.08 days long. Clearly months are not an average of 31.08 days long. But, most people seem to feel more comfortable with 4.44 as a conversion factor because the calculations seem to make more sense. You sometimes have to ignore facts and go with what makes people feel good. In summary, care providers don't use months because they are not clinically useful, there is no agreement on how long a month is, and the math is too complicated. If you want to convert weeks gestation to months gestation divide the weeks by 4.35 weeks per month if you assume that a pregnancy is 9 months and 6 days long (which it is) or by 4.44 weeks per month if you prefer to think that a pregnancy is exactly nine months long.


Evonna - April 11

my eyes hurt..


AmyF - April 11

Holy mother- this is absolutely crazy! LOL . Dh would enjoy the reading. I'm forwarding this to him- he'll get a kick out of figuring the math.


EM - April 11

Holy cow I quit reading about a quarter of the way down but I always found it amusing to ask people so, how many months is 40 weeks? They almost always reply I say, we're really pregnant for 10, not 9 then!


Chrissythefairy - April 11

yeah im lazy too cause i certinly didnt read all that but yeah i usually say how many weeks i am anyway cause the month thing gets real comfusing to people


HannahBaby - April 11

i always say weeks and most ppl say "well how many months is that" and i always say, duno, my dr goes by weeks. However, the what to expect when your expecting books breaks down what weeks fall in what month


GraceRenee - April 11

Ha ha.. that is great. Being a math minded person I love stuff like this. You're right weeks is so much better. Whenever I talk to a pregnant person, I ALWAYS ask them "How many weeks are you?" It is funny cuz sometimes they are like me and have it down to weeks and days, and sometimes they have no idea whatsoever. We are all so different!


Mandy1984 - April 11

its dead easy to work out how many months pregnant you are, you are pregnant for 9mths NOT 10 which is 40 weeks.. eg I am14++ weeks pregnant which most people would say 3 and 1/2 mths, but I'm due on 9th october so even though 14 weeks I was only 3mths pregnant on 9th october, will be 4mths pregnant on 9th May,( 18weeks) 5mths pregnant on 9th June (22wks) ect.... So it is easy to use 'months' when pregnant, but not going by how many weeks you are but by the present date and the date your due...If that makes sense?? :)


Mandy1984 - April 11

''correction'' 3 mths pregnant on the 9th April


Remy - April 11

Thanks Steph, I finally understand what all the controversy is about now. I think I'm gonna stick with weeks. =)


GraceRenee - April 11

Interesting method Mandy... when I figure months I always go from LMP just like with weeks. So my LMP was on Jan 13, so I am 3 months on April 13, 4 months on May 13 etc. It is kind of like splitting hairs, but it does make a slight difference... so I think that is the reason why Drs. use weeks since LMP to date the pregnancy... that way they can be sure everyone is "speaking the same language".



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