How to Calculate the Number of Weeks Pregnant

Why the Number of Weeks Matters in Pregnancy

Pregnant woman figuring out how many weeks pregnant she is...
Photo © Sydney Bourne/Getty Images

Due date! It's what everyone wants to know from the moment you pee on a stick. When are you due? When can we expect the baby? When will your baby be born? Will your baby come early? When will they induce you? The questions fly at you!

The more important thing to know is how many weeks pregnant you are at any given point in pregnancy. This is how doctors and midwives calculate your pregnancy, rather than months, because it's more accurate and can give your practitioner a better idea of exactly what test results should look like, which tests might be most appropriate, and yes, when your baby may be born.

Your baby is growing so rapidly, particularly in the first weeks of pregnancy that a whole week can mean vastly different things when it comes to not only fetal development but also with treatment of various pregnancy issues. This is why it is important to use a method of calculating how far along a pregnancy is that can be fairly accurate and go across multiple practitioners - a plan everyone understands, so to speak. This is how we came up with using the number of weeks, as opposed to the number of months. 

How to Calculate the Number of Weeks Pregnant

You can calculate this date using a simple formula and a piece of paper and calendar. To calculate the number of weeks you are, you simply start with the first day of your last menstrual period, also called LMP. So if the first day of your last period was January 1st, your first week of pregnancy is January 1-7th, though you are one week pregnant on January 7th as you complete the week. January 8-14th would be the second week, two weeks pregnant on January 14th. Your doctor or midwife may even count days.

So for example, if you were, using the January 1st date, five weeks three days pregnant would be February 7th. If you were doing an early ultrasound to look for a heartbeat, you may not see much of anything, but wait a week, until after six weeks, around February 15th, and you would see a lot more. This is a good example of how many changes can take place within one week of pregnancy and helps strengthen the case for using weeks over months.

Alternatives to Using a Calendar Method

Don't get stressed about the details, there is a really easy tool to use that will tell you each personalized week of your pregnancy, including when you can expect to hear the heartbeat and feel your baby move. Simply put your due date into this calculator: Pregnancy Weeks Calculator It's a good idea to keep that info handy. If someone ever asks you have many weeks you are, you can quickly and easily tell them.

You may also have seen a gestational wheel calculator. These are pretty commonly found at prenatal appointments. You simply swivel the wheel to either the due date you have been given or the LMP you're using and then look at what it says for the date you're looking up or the due date. There are also a number of apps that you can use to help you do that. 

If your grandma insists on knowing how many months you are, you can use the calculation to figure that out as well. (You'll see why it's a bit less straightforward than weeks and why clinical personnel dropped it.)

So, how many weeks are you? If you haven't figured it out by now, make sure you make a note to ask your midwife or doctor at your next appointment.

View Article Sources
  • Committee opinion no 611: method for estimating due date. Obstet Gynecol. 2014 Oct;124(4):863-6. doi: 10.1097/