Everything Revolves Around Your Due Date! How Is This Magic Number Calculated?
How is a due date calculated?
A due date calculation is relatively simple if you know the first day of your last menstrual period (LMP). It is based on the average length of a human pregnancy -- 280 days, or 40 weeks, from your LMP. It also takes into consideration that the average menstrual cycle is 28 days and a luteal phase (the period of time from ovulation to menstruation) is 14 days. The good news is that most due date calculators (such as the one right here at Pregnancy & Baby) allow for you to adjust for cycles that are longer (or shorter) than the average, and can even allow you to specify the length of your luteal phase.
When you visit your healthcare provider, he or she will likely pull out a little wheel to use to calculate your due date. This also uses your LMP but might not have the ability to adjust to your exact specifications (for example, a 32-day cycle with a 15-day luteal phase).
Why it's a due "month"
In essence, getting your due date pinpointed exactly based on your specific ovulation and conception date might not be so important after all. The truth is, your baby will be considered full term starting at 37 weeks -- a full three weeks before the actual due date! This means that she will make her appearance at any time starting then, although your chances of having your baby increase as the days go by. From 37 weeks to all the way to 42 weeks, you can be reasonably assured that your baby will arrive. Babies are really not "early" nor "late" unless they are born before or after this time period.
There are a few reasons why your due date might be less obvious than it should be. One would be if your periods are irregular (for example, your cycles vary in length by as much as a few days to a few weeks) your due date could possibly be off by a week or more. This is important because after 42 weeks of pregnancy you are considered overdue and your provider might begin to talk about inducing your labor. Inducing your labor when you really aren't overdue might be dangerous for a baby who isn't quite ready to come out. In a case like this (when your dates aren't clear) you might have an early ultrasound to help determine how far along you are.
Haven't you had that baby yet?
Waiting for your baby's arrival can be really trying. Not only are you eagerly awaiting your baby's birth, so is everyone around you! Even if you know that you can go to 42 weeks with no adverse health problems, you will likely start to feel more anxious as your due date approaches. The first time you buy a carton of milk that has an expiration date that is past your due date you might be tempted to cry.
Have faith, though, that having good prenatal care with a trustworthy healthcare provider will alleviate your concerns and help you make the best decisions for your (and your baby's) health. In the meantime, let someone else do the grocery shopping for you!
- Due date calculator
- Dealing with the overdue blues
- What month are you due? Join a Pregnancy & Baby due date club!