How off can your due date be

You have just attended our centre for an early pregnancy scan and your due date doesn’t match your last menstrual period (the LMP) and you are very certain of your dates.

Your due date is important for many reasons. It will allow you to plan ahead for your baby’s arrival and it also helps your doctor know when certain prenatal tests should be performed to make sure your baby is developing normally. With so much being based on your expected due date it’s really important for it to be as accurate as possible. 

A gentle warning is that menstrual cycles are seldom that regular. Normal cycles can be between 21 to 35 days with some women outside of this guideline. Regularity is seldom that common and there is a fair chance the calculation will be off. But we still use your LMP as a guideline because the beginning of a period is an identifiable event where ovulation and fertilisation is not. It is very common when scanning an early pregnancy to find that the due date does not match the menstrual history. Sometimes the dates can be more than a week off and sometimes even as much as 4 weeks.

An early obstetric ultrasound performed at approximately 8 weeks where a crown rump length (the CRL) is measured precisely predicts an estimated due date accurately. The accuracy of ultrasound measurements used to confirm the calculated due date will be much less accurate further into the pregnancy.

On an early pregnancy ultrasound the embryo’s crown rump length is used to predict your estimated due date ONLY if the measurement falls outside of what your last menstrual period predicts. Measuring the embryo on ultrasound is accurate to within plus or minus five days of the due date. If the crown rump length falls outside of these five days we give you a new estimated due date based on the size of the embryo.

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All of the content and articles on our blog and website are intended for informational purposes only. Please do not consider any of the information provided here as a substitute for medical advice. At all times seek medical advice directly with your own doctor and medical team.

Throughout my pregnancy, I've received two different due dates — one came from my very first ultrasound, when my baby was barely the size of a blueberry, the date that was then subsequently stamped on my record as "the" due date.

But then there was a second date, the one that showed up on my next couple of ultrasound scans. This date was a few days earlier. It's also the date that I prefer, not only because I'm now 38 weeks pregnant and ready to get this baby out, but because it matched the date I'd calculated on my own using information I looked up.

But ... which one should I believe? And how much does it really matter? After all, as many moms know, due dates are just educated guesses.

How are due dates calculated?

Traditionally, estimated due dates are calculated by taking the start date of a woman's last menstrual period and adding a year, subtracting three months and adding seven days, a method known as Naegele's rule.

"If you're a woman whose period is like clockwork, it's pretty accurate," Dr. Tamika Auguste, MD, an OB-GYN at MedStar Washington Hospital Center in Washington, D.C., told TODAY Health.

But that method doesn't account for irregularities in a woman's menstrual cycle — if she has a longer or shorter cycle than 28 days or any variability with ovulation, for example. That's why the American College of Obstetricians and Gynecologists recommends doctors use ultrasound measurement in the first trimester to establish an estimated due date.

The most accurate way to date a pregnancy is with an ultrasound done between 6 weeks and 8 weeks, 6 days, according to Dr. Michael Cackovic, MD, an OB-GYN with The Ohio State University Wexner Medical Center.

"The reason why is there's very little variation that can be detected in a measurement from that time," he said. "Pretty much all pregnancies are equal at that stage."

As a woman's pregnancy progresses, fetuses develop at different rates, which is why an estimated due date from an ultrasound measurement during the second trimester is less accurate. (And why I probably shouldn't be taking my second "due date" very seriously.)

As much as technology has improved, this is all still an imperfect science.

"Often we get a due date based on the last menstrual period and then do a sonogram and there's a discrepancy we have to address," Auguste added. "If it's all within a week, it's consistent. Once you go past about five or six days, then you have to start considering that your dates really may be off."

How reliable is my due date?

Even once your doctor has established a due date, there's no guarantee that your baby will come that day. In fact, only 6% of women deliver on their due date, Cackovic said.

I tell patients that really, your due date is a date in the middle of almost a month-long period of time that you could have your baby.

Dr. Michael Cackovic

"The reality is that about 70% of women will have their baby within 10 days of their due date," he said. "I tell patients that really, your due date is a date in the middle of almost a month-long period of time that you could have your baby."

Nearly 10% of babies in the U.S. were born before 37 weeks of gestation in 2017, according to data from the Centers for Disease Control and Prevention, while 26% were born between 37 and 38 weeks and 57% were born at full term between 39 and 40 weeks. Some babies arrived after the due date, too: another 6% of babies were born in week 41 and less than 1% at 42 weeks.

Are there signs my baby will come early ... or late?

There are many signs that labor is coming: The cervix softens, a woman may feel that her baby has "dropped" lower in the pelvis or she may experience what's known as "bloody show," discharge that contains mucus and blood.

While these are all signs labor will happen — eventually — there's just no way to know when, exactly.

"The bottom line is we just don't know what starts labor," Cackovic said. "People ask me this all the time. The reality is that it's some complex mechanism between the baby's brain, the mom's brain and the placenta and we haven't figured it out yet."

There are some rules of thumb that OB-GYNs follow, though.

"Women who have had multiple babies typically deliver earlier with each subsequent pregnancy," said Dr. Shannon Clark, MD, a professor of maternal-fetal medicine at the University of Texas Medical Branch at Galveston and the founder of BabiesAfter35.com. "First-time moms typically go to their due date in an otherwise uncomplicated pregnancy."

What about routine cervical checks?

Around week 36 or 37 of a woman's pregnancy, some doctors will begin to offer cervical checks at appointments, to see if a woman's cervix has started to dilate (open up) and efface (thin out).

"First-time moms typically efface first, and women who have had more than one delivery typically dilate first," Clark said. "Unfortunately, this does not really tell us when she will go into labor. What we as providers are looking for is some change in the cervix, whether in dilation or effacement, so we will know things are progressing in the right direction."

Some doctors may not recommend routine checks, and some women may choose to opt out of them, because while it can be tempting to have the information, being dilated or effaced doesn't tell exactly when the baby's going to come.

"I don't check anybody's cervix unless they ask me to, or if they're contracting," Cackovic said. "Medically, there's just no reason to do it."

While there's no shortage of tips to kick-start labor available on the internet, Auguste stressed that it's important to talk to a doctor about the signs of labor before trying any natural induction methods.

"A lot of the information out there is old wive's tales or hearsay, so you really need to have that conversation with your provider," she said.

How many days can a due date be off?

Can due date be off by a month?

Can gestational age be off by 2 weeks?

Why would my due date be off?