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Are you pregnant, or know someone who is? Before you ask when the baby is "due", ask what estimated due dates really mean. A new study just found that pregnancy length naturally varies by as much as five weeks.

“When is the baby due?”

This is the first question many parents to-be ask themselves after that pregnancy test came up positive, and the one question that everyone — from close relatives to total strangers — will be asking them for the next... roughly 40 weeks.

The baby's estimated due date (EDD) or the mother's estimated date of confinement (EDC) is a central theme of the first prenatal appointment and something that will be reviewed during each subsequent prenatal visit.

So, “when is the baby due”? The estimated due date is traditionally calculated by taking the date of the mother's last menstrual period (LMP for short) and adding 280 days, or 40 weeks. It is also possible to calculate the due date more exactly if the date of conception is known, or in the case of IVF by looking at the date of the three- or five day blastocyst transfer.

When the precise date of conception is known, the due date is calculated by adding 266 days to it.

Some pregnant women don't recall when their last menstrual period took place, or they have irregular cycles to the point that the date of the last menstrual period doesn't matter much, because it says nothing about the probably conception date. In these cases, the baby's estimated due date is determined on the basis of an ultrasound scan. In the early weeks of pregnancy, this method of calculating gestational age is very reliable — even more so than going by the date the mom's last period started. 

Why We Have Due Dates

Due dates certainly have their use. In a pregnancy that progresses normally, a due date gives doctors and parents an idea as to when the baby may make its appearance.

But more importantly, the due date helps detect abnormal situations — those that require intervention or additional care. 

In current medical practice, a pregnancy is considered to have reached full term at 37 weeks. If a mother goes into labor before that time, the birth would be premature. Sometimes, steps can be undertaken to prevent premature labor from progressing. When this is not feasible, steroids can hasten the baby's lung development to limit damage, and neonatal intensive care units provide the best possible care. 

More recent research shows that babies who are born between 39 and 41 weeks have a lower risk of newborn death than those born at 37 or 38 weeks. This suggests that we made need to rethink what "full-term pregnancy" really means and that babies born prior to 39 weeks may require more care than we generally provide. 

Meanwhile, being "overdue" or pregnant beyond 42 weeks also comes with risks. After 42 weeks, the risk of stillbirth rises considerably and a Dutch study published in 2012 also shows that these babies are much more likely to have behavioral problems — including ADHD — later in life. 

Having an estimated due date helps medical professionals make judgments that can save lives, but an estimated due date is not an "eviction date" and remains nothing but a rough estimate. Do you know how many babies are born on their due date, and how big the variations in natural gestational length are? We will discuss that in the next section. 

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