What Percentage of Babies Are Born on Their Due Date
How Realistic Is Your Due Date?
A due date assumes pregnancy will last about 40 weeks, but it's not meant to be a deadline for delivery.
This story was originally published on Aug. 27, 2019 in NYT Parenting.
When I was 37 weeks pregnant with my first baby — three weeks before my due date — my obstetrician did a routine check of my cervix and noted that it was starting to dilate and shorten, signs that the wheels of the long birth process were beginning to turn. "You need to be prepared for labor to start any day," I remember her saying, adding that I might have a baby within a week. I can't remember if she emphasized the word might while reminding me that babies and birth were unpredictable, but if she did, I disregarded that and took her best guess as near-fact.
Ripe with the naivety and anticipation of a first pregnancy, I sprang into action. I installed the car seat, packed a hospital bag and scrambled to finish work projects. And then I waited. The days and weeks ticked by and my due date passed uneventfully. As my belly grew, so did my discomfort and impatience, until my daughter finally made her appearance — five days after her due date.
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Had I read the research on pregnancy length, instead of frantically preparing for birth and taking long walks to try to start labor (which hasn't been proven to work), I might have appreciated how much pregnancies can vary, and how difficult it is to guess when a baby will be born. In fact, while I thought of my daughter as being late and overdue, like a library book racking up fines with each passing day, her arrival was well within the realm of normal.
A due date assumes pregnancy will last about 40 weeks, but it's not meant to be a precise prediction or a deadline for delivery. A 2013 study of about 18,700 women in Australia, for instance, found that just 5 percent of births happened on their due dates.
"It's an estimated time for the birth of your baby, with a big emphasis on estimated," said Lisa Kane Low, Ph.D., a certified nurse midwife and a professor at the University of Michigan School of Nursing. Giving birth up to two weeks before and two weeks after are still considered normal, she said, though she noted that as inductions have become more common, it's rare for pregnancies to last 42 weeks.
According to data from the Centers for Disease Control and Prevention, about 10 percent of the 3.8 million babies born in the United States in 2017 came preterm (before 37 weeks). Twenty-six percent were born in weeks 37 to 38; 57 percent in weeks 39 to 40; 6 percent in week 41; and less than 1 percent at 42 weeks or beyond. In 2017, 73 percent of babies were born before their due dates. Two decades earlier, in 1997, that figure was 57 percent. That difference is partly because inductions and cesarean births have become more common, but also because methods for estimating due dates have improved.
Still, predicting due dates is an imprecise science, largely because we rarely know exactly when pregnancy begins, which means there's a lot of guesswork involved. Due dates are estimated by taking the first day of the last menstrual period and adding 280 days. But this assumes that we all have cycles lasting exactly 28 days (we don't), that ovulation always happens on the 14th day (it doesn't), and that we can accurately remember our last period (nope, not always). People who conceive with I.V.F. have more precise information about pregnancy timing, which is used to estimate their due dates, but even then, exact predictions are shaky.
With the exception of I.V.F. pregnancies, the American College of Obstetricians and Gynecologists says that fetal measurements taken during a first trimester ultrasound are the most accurate way to estimate a due date, especially for people with irregular menstrual cycles. Sometimes, initial due dates are revised after this ultrasound. An ultrasound done in the second or third trimester is less accurate for estimating due dates, because fetal growth becomes more variable as the pregnancy progresses.
For practical purposes, due dates allow expecting parents to plan for parental leave, child care, and the travel plans of family and friends who might come to help after the birth. But from a medical standpoint, they're vital to tracking the progress of the pregnancy. "The tests we order, the counseling we give, the discussions we have are often based on the pregnancy time point in weeks, so having an accurate and unchanging due date is helpful," said Dr. Christian Pettker, M.D., chief of obstetrics at Yale School of Medicine and one of the authors of ACOG's guidelines on estimating due dates.
Even if you carefully tracked ovulation and know when your baby was conceived, your due date is still an estimate, because every pregnancy is different. That was demonstrated in a 2013 study in which researchers estimated the due dates of 125 women who were trying to conceive in the United States. They pinpointed the days they had ovulated by testing their urinary hormone levels and then followed their pregnancies. "What's really cool is that even with this exact date, there was still five weeks of variability in length of pregnancy," said Anne Marie Jukic, Ph.D., an epidemiologist at the National Institute of Environmental Health Sciences who led the study.
Observational studies have tried to tease out factors that might explain some of this variability. For example, one study of more than 40,000 women in London published in 2016 found that if a woman's first baby came before or after her due date, her second baby tended to do the same, but not by as many days. Another study published in 2006 looked at more than 77,000 couples in Norway and concluded that gestational lengths might be inherited: meaning that the amount of time your child develops in the womb might be similar to the amount of time that you spent in your mother's womb. And another study of about 119,000 women in Northern California found that those who were on their first pregnancies or who were obese were more likely to deliver at 40 weeks or beyond, while those with complications like high blood pressure or diabetes were more likely to deliver before their due dates.
But by their nature, observational studies can show only statistical correlations; they can't demonstrate cause and effect. Additionally, much of this research — and thus our understanding of pregnancy length and factors influencing it — has been conducted in white populations and may miss important factors influencing pregnancy, and by extension, maternal and infant health. "We're looking at research in incredibly homogenous populations that may not reflect the diversity in any one community," said Dr. Amanda Williams, M.D., an ob-gyn and maternity director at Kaiser Permanente Oakland Medical Center in California.
Older studies in both the United States and Britain have found that a pregnancy's length can vary with race and ethnicity, where white women tend to have longer pregnancies and are more likely to reach their due dates than black or Asian women. Black women in the United States are also at greater risk for preterm birth, which contributes to a higher rate of infant mortality among them. These outcomes are most likely caused at least in part by social inequality, the chronic stress of experiencing racial discrimination and, as reported in the journal Pediatrics in August 2019, disparities in NICU care.
Because of the data limitations on pregnancy length, Dr. Williams said she doesn't dwell on the precision of due dates and the many factors that might nudge a baby to come a little earlier or later, explaining that they're unlikely to be clinically significant for individual patients. "What I do tell them is that risk of preterm birth in African-American patients is much higher, so we're going to take contractions much more seriously," she said.
And when she's weighing decisions like whether to induce labor, she's not just thinking about the due date but other factors that are known to increase the risk to the baby as pregnancy continues, like diabetes or high blood pressure. "Medicine, especially pregnancy management, is as much an art as it is a science, and we have to individualize our care and take as much information as possible about that person and that pregnancy as we're making decisions," Dr. Williams said. "There are very few absolutes in obstetrics."
I know I wished for more absolutes during the weeks of waiting for my daughter's birth. I was used to having more control over my schedule, and it was hard for me to let that go. But in hindsight, that period was a fitting introduction to the unpredictability of babies and the patience necessary for parenting. Eight years later, my daughter still makes me wait daily: "Hang on, Mom, I'll be there in a minute!"
Alice Callahan is a health and science journalist, a mom of two and the author of "The Science of Mom: A Research-Based Guide to Your Baby's First Year."
What Percentage of Babies Are Born on Their Due Date
Source: https://www.nytimes.com/2020/04/17/parenting/due-date-accurate.html
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