How Doctor’s Calculate Your Due Date


From the moment a woman first sees those two lines on a pregnancy test, a lot of thoughts run through her mind, and one of them is when her baby will be due. Having a due date can be exciting, and it can help mom start planning what she is going to do and start organizing for when the baby’s big arrival will be. Mom may start doing the math in her head trying to figure out when the baby will come, but there are ways that are more definite than doing “period math” to try and narrow it down.

The first thing a woman does when she finds out she is expecting is make a doctor’s appointment. This is when the pregnancy will be confirmed with blood work, and a lot of other prenatal tests will be done. This is just the start of her pregnancy journey, and while there will be many exciting moments along the way, finding out the due date will always be one of the most exciting. However, it is typically her doctor who will give her these due dates, and they do this in two main ways.

RELATED: How Often Are Due Dates Wrong And Why

Date Of Last Period

According to Johns Hopkins Medicine, the doctor may be able to give you an estimated due date at your first appointment and they will often go off the first day of your last menstrual cycle. While it may seem odd, the first day of your pregnancy will actually be the first day of your period. That’s right, even though you hadn’t even conceived yet, this will be the beginning if your journey. Since pregnancy lasts 280 days (40 weeks) they will count based on that date. However, not all women have regular cycles, so this is going to likely be a very vague estimate. So, how do they narrow it down for sure to receive the final date?

A Dating Scan

A dating ultrasound is much more accurate at determining what your due date is, according to Healthline. This will typically be what your doctor relies on 100% to determine how far along you are and therefore when your baby is due. They will order an ultrasound and they will measure the baby’s size from crown to rump. That is the length of the fetus. This is the most accurate way to determine gestation, especially for women who have irregular periods. While this due date will likely be different than the date given based on the last menstrual cycle, it is not usually far off.

Can The Doctor Change My Due Date?

Some women may be wondering if their doctor will change their due date throughout their pregnancy. According to Very Well Family, while this can happen, it is not very common. Typically, the due date you are given after a dating ultrasound will continue to be your due date for your entire pregnancy, but there are certain factors that could cause a doctor to change their mind. The fundal height measurement throughout pregnancy (the measurement to the top of the uterus), could indicate that you are further along than believed. It is also important to note that if a due date changes, they are usually only a few days off in either direction.

Sources: Johns Hopkins Medicine, Healthline, Very Well Family

Why Your Pregnancy Isn’t Likely To End On Your Due Date

Here’s why your pregnancy isn’t likely to end on your due date.

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