The Due Month

“When are you due?” This is probably the question expectant moms get asked most frequently. Often, they will simply quote the "due date" that they have been given by their health care provider. But what many childbirth professionals are realizing is that it actually might make more sense to talk about a “due month” rather than a “due date.” 

Research supports a more accurate way of predicting when babies will arrive is a span of weeks. Being aware of this can help parents have more flexible expectations and lower stress levels at the end of pregnancy.

Let’s take a closer look at the concept of due month vs. due dates, including how due dates are generally calculated, when babies are typically born, and why the idea of due months may have advantages over due dates.

How Are Due Dates Calculated?

There are two primary ways that due dates are calculated: based on last menstrual period (LMP) or based on ultrasounds and other diagnostic tools during pregnancy.

Due date calculations using LMP are based on the idea of pregnancies lasting 40 weeks from that date, which comes out to 280 days. The starting date is calculated using the first day of the LMP and counting to 40 weeks from then.

One problem with this method is that most pregnancies don’t last exactly 40 weeks. Another issue is that a woman may not remember the exact date of her last period. She might be confused if she experienced spotting during ovulation or after conception. 

Finally, having an irregular period can also throw things off. Due dates calculated by LMP are based on a 28-day cycle and assume that ovulation and conception happen on day 14. But many women have cycles that are shorter or longer, or a length that varies from month to month. Any of these variables will result in an inaccurate calculation using LMP.

When the timing of the last menstrual period is unclear, health care providers may use ultrasound to determine or confirm a due date. They may also use uterine measurements at different points during pregnancy. No matter how a due date is calculated, it is important for parents to understand that it is only an educated guess. Even when the due date is known to be accurate from ovulation, length of pregnancy still varies considerably.

How Accurately Do Due Dates Predict Labor?

Not only are due dates difficult to calculate precisely, but babies are rarely born on their due dates anyway. So when are babies actually born?

Research has found that roughly 60% of women give birth on or prior to their due dates. In about another 35% of women, labor starts on its own within two weeks after the due date. But 5% of women don’t start labor until after 42 weeks. Other research has found that only about 1 in 20 pregnant women, or 4% actually give birth on their due date.

All of this is why some experts think it makes more sense to look at a due month—or a 4–5 week span—rather than a specific due date. Plumtree Baby's parent book Preparing for Birth explains the due date dilemma in detail.

Disadvantages of Focusing on Due Date

While due dates can help parents prepare for birth and track their baby’s growth progress, focusing on that one specific date can actually be stressful.

First of all, it’s unlikely that birth will happen on the due date. There is a five-week range around the due date that is healthy. Focusing on one specific date can create panic and distress for expectant parents if it passes without a baby. This may be compounded by well-intended friends and family repeatedly asking if the baby is here yet. Who needs that kind of pressure?

Another issue that can come up is that if parents are zeroed in on their due date, they might be taken aback if their baby arrives a few weeks early. Focusing on a due month instead can help them prepare their space, gather all needed baby supplies, and get in the mindset of welcoming their baby anytime within the span of that month.

Finally, if expectant parents and their care team have a narrow view of when the baby should arrive, it’s possible that there will be a higher risk for labor interventions, such as induction.

Inducing labor for medical concerns may be necessary, but induction based on a due date alone may lead to unnecessary labor inductions. Usually, there is no significant harm in waiting a few extra days before considering induction, even when the due date has passed. When parents are encouraged to focus on a range rather than a specific date and keep their expectations more expansive and flexible, they are even less likely to run into unnecessary interventions. This education about due dates empowers parents.

Of course, not all providers will take this attitude. But encouraging parents to discuss a more open-ended due date with their health care provider may help them to consider medical reasons for inductions, such as baby’s growth or health status, rather than focusing on the date alone. It’s important to avoid unnecessary inductions because they can increase the risk of medical problems during labor, lead to more labor interventions, and increase the likelihood of prematurity.

The Bottom Line

Researchers agree that there are advantages to focusing on due months rather than due dates. Journalist Rachael Rettner reports: "'The emphasis on a single due date may make the length of pregnancy seem more predictable than it really is,' Jukic said. Providing women with a range of due dates may be a better way to communicate the length of pregnancy." Educating parents helps them understand that their baby likely will not be born on that date and allows them to prepare for all eventualities.

Sources

• Institute for Quality and Efficiency in Health Care (IQWiG). Pregnancy and birth: When your baby’s due date has passed.
• March of Dimes. Medical reasons for inducing labor.
• Norwitz ER. Patient education: Postterm pregnancy (Beyond the Basics). UpToDate.
• LiveScience: Length of a Healthy Pregnancy Surprisingly Variable.

Wendy Wisner, Freelance Writer and Lactation Consultant (IBCLC)

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