Calculating Weeks and Months

You may wonder how pregnancy can be said to be both nine months and forty weeks long. Forty weeks sounds like ten months! How is that?

  • Counting weeks: The 40 weeks we estimate for pregnancy actually begin at the day of the last period, on average two weeks before you even conceived! This tradition began decades (maybe centuries) before ultrasound and ovulation detection were invented, when all they had to go on was the date of the last period. So don't be surprised if your practitioner tells you that you are eight weeks pregnant and you know you only conceived six weeks ago. Even in vitro (IVF) pregnancies are dated like this-knowing the exact date of conception doesn't change the way pregnancy weeks are counted. Ultrasound results are adjusted to this method too, so if your sonogram says the embryo is eight weeks in size, it is just six weeks after conception. Confusing, but that's the tradition.

  • Counting months: Since pregnancy lasts about forty weeks, you can't just sum up each four weeks as a month, or you would be ten months by your due date! Is 32 weeks 8 months (four weeks times eight) or 7 months (eight weeks left in the pregnancy)? You can see why medical personnel always count in weeks. But I finally figured out a way of calculating months. Just count backwards from your due date-so if you are due July 15, that will be nine months, you will have completed eight months on June 15 and seven months on May 15, etc.

  • Traditionally you only get "credit" for time after it is past. So you are nine months on your due date, and "in your ninth month" the previous 30 days. You are 40 weeks on your due date, and in your fortieth week the previous seven days. This may seem confusing, but it is the same way that we judge someone's age-your baby will be one year old on the first birthday, and in his/her first year until then.