How Long Should Pregnancy Last?

“When is the baby due?”

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

So, “when is the baby due”? A baby's 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, to that date. It is also possible to calculate the due date more exactly if the date of conception is known, or in cases where a couple used IVF to get pregnant, by looking at the date of the three- or five day blastocyst transfer.

Some pregnant women don't recall when their last menstrual period took place, or they have irregular menstrual cycles to the point that the date of the last menstrual period doesn't matter much, because it says nothing about the probable conception date. In these cases, the baby's estimated due date is determined on the basis of an ultrasound scan, which enables the ultrasound technician to estimate the fetus' gestational age quite accurately. 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, in fact. 

Why Do Due Dates Even Exist in Pregnancies?

Due dates certainly have their use. In a pregnancy that progresses normally, a due date gives doctors and parents alike an idea as to when the baby may make its appearance, which allows both to prepare for the baby's birth.

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 considered premature. Sometimes, medical steps can be undertaken to prevent a premature labor that has already started 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. In both cases, these steps can make a significant difference for the baby's health outcomes.

More recent research shows that babies who are born between 39 and 41 weeks gestation 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", classified as being pregnant for longer 42 weeks, also comes with health risks. After 42 weeks, the risk of stillbirth rises considerably and a Dutch study published in 2012 also shows that these babies who are born "too late" 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. 

The Problem With Due Dates

We know that babies who are born prematurely are much more likely to have medical problems, and we also know that it is safer to take steps to induce labor than to let a pregnancy continue beyond 42 weeks. But how much does a baby's assigned estimated due date really say about the day on which he or she is likely to be born?

We already knew that very few babies are born right on the due date they were given. If you are going to place a bet on your baby's birth date, your odds will be much better if you do not pick the actual due date. 

Pregnancy length appears to vary quite a bit naturally, but how much? Previously, the apparent variations in gestational length were sometimes attributed to faulty due date calculations — in other words, perhaps the mother got the date of her last period wrong. A recent study of 125 women set out to discover natural variations in pregnancy. In this case, the researchers knew the exact dates of conception.

The research team from the US National Institute of Environmental Health Sciences took daily urine samples from women who were trying to conceive naturally in order to find out their ovulation and implantation dates. They were surprised by their own findings:

• Gestational length naturally varied by five weeks (37 days), even after excluding six premature births.

• The average gestational length, measured from conception, was 268 days or 38 weeks. 

• Embryos that took longer from conception to implantation also gestated for longer.

• Older mothers were more likely to have longer pregnancies. 

• There was a link between gestational length and maternal weight. 

What can women who are having babies now take away from this study? Well, to start with, that there is really no need to be induced simply for gestational age once you reach 38 weeks.

The estimated due date really is no more than a somewhat informed estimation. Perhaps pregnant women can learn something from Kate Middleton, who did not offer the public an exact due date. When people ask you when you are "due", you can offer a month-long window instead of an exact date — as in "sometime in June, or maybe early July". You'll be giving a more accurate answer that way, and you will avoid those annoying "is the baby there yet" calls right around your due date.