All About Due Dates

Why Do You Use Weeks, Not Months?

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. 

If you're pregnant and want to know your estimated gestation in weeks or months, try the Gestation Calculator.