How Is My Due Date Calculated, and How Does It Relate to My Prenatal Testing Timeline?

07/21/2016

Written by: Jenny Daniels

One of the first questions most women ask when they find out they’re pregnant is, “when’s my due date?”

Although this is an exciting time to start preparing for the arrival of your baby, due dates are calculated right away for another very important reason. Throughout your pregnancy, you will be given options for prenatal testing, which will analyze the health of your baby. Each test has a timeline of when it can be performed in relation to the gestational age of the baby.

How Is My Due Date Calculated?

Your physician will calculate your due date by counting 280 days from the first day of your last period. This is assuming that you have an average menstrual cycle of 28 days. However, every woman is different, and the length of your menstrual cycle will add some variance to the arrival day of your baby.

To calculate the length of your menstrual cycle, note the first day of your last menstrual period. Then, count the number of days that passed until the first day of the following menstrual period. This number is the length of your cycle.

Your physician may also conduct an ultrasound each trimester to confirm the gestational age of the baby or if the date of your last menstrual period is not known.

This Is Only an Estimate

About 5% of women give birth on their due date. It is difficult to calculate an exact due date because it is often difficult for a couple to know exactly what date conception occurred. For this reason, physicians count from the first day of your last menstrual period.

Although few women actually give birth on their calculated due date, it is important for your doctor to determine a due date that is as accurate as possible in order to adhere to any timeline regarding necessary or optional testing.

How My Due Date Affects the Prenatal Testing Timeline

Your obstetrician will be closely monitoring the health of your baby throughout your pregnancy. Part of this process may involve prenatal testing. Obstetricians are offering women the option of prenatal testing in order to analyze the risk that their baby has a chromosomal disorder or a neural tube defect. These tests are not mandatory, and it is your choice to decide if they will be conducted.

There are multiple tests available including non-invasive prenatal genetic testing, which involves a simple blood draw from the mother, and invasive procedures such as CVS and amniocentesis.

Each test has its own timeline and restrictions as to when it can be performed. For example a mother with an elevated risk of having a baby with a chromosomal abnormality may prefer to be tested as soon as possible – this allows her more time to prepare for the arrival of her baby. In this case, she may opt for non-invasive prenatal screening, which can be performed as early as week 10 in her pregnancy.

For individuals whose prenatal screening results suggest an elevated risk of a chromosomal abnormality, the next step might be to perform CVS or amniocentesis. CVS may only be performed between week 10 – 13 while amniocentesis can be performed between week 15 – 20 of pregnancy.

Discuss Testing Options With Your Healthcare Provider

It is important to speak with your healthcare provider or genetic counselor about prenatal testing. These healthcare providers will help you understand the timeline of each test as well as answer any questions you may have regarding the process.