Will Knowing My Conception Date Make My Due Date More Accurate?

The short answer is that, yes, knowing your date of conception may be able to make your due date slightly more accurate. However, a due date is still difficult to pin down. This is because it can take a variable amount of time for an egg to become fertilized, and for that fertilized egg to implant and begin to grow, before pregnancy actually occurs.

Generally speaking, your due date will be determined using what is called “Nagele’s Rule.” This method for determining due date was developed by Dr. Nagele in the mid-nineteenth century. Dr. Nagele calculated that the average pregnancy lasted 266 days from the date of conception, or 280 days from the last menstrual period (referred to as LMP). This is where we get the figure that pregnancy lasts for 40 weeks – 40 weeks is exactly 280 days.

The thing is, even using the LMP date isn’t exactly accurate. 280 days is just an average. This can be different based on any number of factors. For example, ethnicity can influence the time it takes for your baby to be due. It can take 288 days, for example, for a first-time Caucasian mother to have her baby. The mother’s age, her nutrition, and even her weight can push the due date one way or another.

One way that some women can get a more accurate due date is by measuring Basal Body Temperature (BBT) and Luteinizing Hormone (LH). Measuring these two factors can tell you exactly when you ovulate. By adding 266 days to the time that you ovulate, you can come up with an even more accurate due date.

There may be other ways to help figure out a more accurate due date, as well. For example, an experienced health care provider may be able to help determine your due date by using ultrasound technology to determine the gestational age of your baby, and thereby calculate your due date. Another way to calculate your due date is to track HCG levels in early pregnancy. For a normal pregnancy, HCG levels double roughly every 2 1/2 days. Your health care provider may be able to perform these tests to help determine your due date.


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