What Is the Most Common Week to Go Into Labor?
Obviously, your due date is pretty important—after all, it’s the ultimate milestone of pregnancy—and the veritable finish line!. But since a due date is more like an estimated date of arrival, it’s normal to wonder: What percent of babies are born on their due date? And what’s the most common week to go into labor anyway? Here, experts answer these questions and more.
As a refresher, a due date is estimated based on the first day of your last period; it also assumes a 28-day ovulation cycle, notes Lisa Jackson, MD, FACOG, an ob-gyn and partner at Uptown OBGYN of New York.
Meleen Chuang, MD, an ob-gyn and medical director of women’s health at the Family Health Centers at NYU Langone, adds that due dates are determined using Naegele’s Rule, which adds 40 weeks (or 280 days) to the first day of your last period. “This is why we try to get the best guess from the patient and follow up the dating with a first trimester official ultrasound,” she explains.
While this ultrasound can be helpful in providing a more accurate estimate based on baby’s size—particularly for women with irregular periods and cycles—a due date may still be subject to change. “The closer we can get to a consistent date, the more accurate the due date will be,” Chuang says.
For first-time moms, pregnancy is typically 40 weeks, measured from the day of her last menstrual period, Chuang explains. This duration doesn’t change with subsequent pregnancies. That said, “patients may go into active labor faster after the first baby, with a shorter total time in labor,” she adds.
So what is the most common week to go into labor? Per Chuang, it usually occurs on or around week 39 of pregnancy. “We always tell patients the timing of delivery is usually plus or minus two weeks from [the estimated due date],” she explains. “Many women can expect to go into labor between 37 to 42 weeks of gestation.”
The number of babies born on their estimated due dates is pretty small—according to both experts, only 4 to 5 percent of babies are actually delivered on their due date. “Most will make their debut within a two-week window on either side of the due date, so it’s unpredictable,” Jackson says.
Baby is considered early, or preterm if they’re born prior to 37 weeks of pregnancy, Jackson notes. Between 8 to 10 percent of babies are born prematurely. But, according to a report from Centers for Disease Control and Prevention] (CDC), there are different classifications within a preterm birth:
- Babies born between 34 and 36 weeks are considered late preterm
- Babies born before 34 weeks are considered early preterm
- Babies born before 28 weeks are considered extremely preterm
Premature birth may happen due to several factors including maternal risk factors, maternal health status, nutrition and a multifetal pregnancy. “Babies born early may need extra medical care, but the level of support depends on how early they are,” Jackson adds.
It’s not entirely clear what “on time” means when it comes to baby’s birth. Jackson and the American College of Obstetricians and Gynecologists (ACOG) note that full-term applies to babies born between 39 and 40 weeks. Between 57 and 60 percent of babies are born between 39 and 40 weeks. (Chuang adds that 70 to 75 percent of babies are born after 37 weeks.)
“A lot of crucial development happens between 34 and 40 weeks, especially in baby’s brain and lungs,” Jackson explains. “While babies born between 37 and 39 weeks are generally healthy, those last few weeks allow for further lung maturity and fat accumulation. This extra time helps them better regulate body temperature and blood sugar after birth, which is why full-term is ideal.”
Late-term pregnancies occur between 40 and 41 weeks, Jackson says, while post-term pregnancies extend past 42 weeks. Between 5 to 10 percent of pregnancies extend past 41 to 42 weeks, experts note—but it isn’t likely you’ll have baby any later than that.
“Most obstetricians will recommend delivery by 42 weeks,” Jackson says. This is because maternal and fetal risks increase past 41 weeks, including a drop in amniotic fluid levels and baby swallowing meconium. These increased risks may warrant interventions such as a c-section delivery or admission to the neonatal intensive care unit (NICU). “These babies will need routine fetal monitoring called a biophysical profile to monitor the well-being of baby once past the due date,” Chuang adds.
Wondering why some moms may go past their due date? Experts note it could be due to several reasons relating to your prior prenatal and cervical medical history, any current medical conditions and baby’s size in-utero.
Frequently Asked Questions
How early can you safely give birth to baby?
Both experts note it’s best to get to between 39 and 40 weeks, as that’s the safest time for Mom and baby. But, generally, babies born after 37 weeks are considered healthy. Plus, “modern medicine has come a long way, and with the right medical support, babies born as early as 24 weeks have a chance of survival,” Jackson says.
Does birth order make a difference in when baby will be born?
“Birth order can have some influence, as studies suggest that first-time mothers may be more likely to go past their due date compared to those who have previously given birth,” Chuang notes. However, Jackson states that birth order probably won’t make a difference in your pregnancy duration. “If your first baby came early, that doesn’t mean that your second baby will come early as well,” Jackson says. “The reverse is also true—if your first baby was late, that doesn’t mean that your second baby will come late as well. In general, you should stay prepared after 37 weeks, and anticipate that baby will come a few days before or after the due date.”
How often is a baby born?
Per Chuang, birth rates vary widely by country. “Birth rates are lower in developed countries. In the United States, the birth rate is steadily decreasing,” she notes. Presently, in the US, a baby is born every 8 seconds, Jackson says, while around 258 babies are born every minute on a global scale.
When you’re counting down the weeks to meet baby, it can be frustrating not to have an exact timeline of when things will get moving. “The main takeaway is that every pregnancy is unique,” Jackson says. “While statistics and trends can give you a sense of what to expect, every baby follows their own schedule. The key is to stay informed, keep an open dialogue with your healthcare provider and remain flexible as your due date approaches.”
Please note: The Bump and the materials and information it contains are not intended to, and do not constitute, medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.
Plus, more from The Bump:
Meleen Chuang, MD, is an ob-gyn and medical director of women’s health at the Family Health Centers at NYU Langone, as well as the chief of obstetrics and gynecology at NYU Langone Hospital in Brooklyn, NY. She earned her medical degree from SUNY Stony Brook.
Lisa Jackson, MD, FACOG, is an ob-gyn with over 15 years of experience and a partner at Uptown OBGYN of New York. She earned her medical degree from Vanderbilt University School of Medicine in Nashville, Tennessee and completed her residency in Chicago at Rush University Medical Center. She also serves as an assistant clinical professor of ob-gyn at the Mount Sinai Hospital in New York City.
Centers for Disease Control and Prevention, National Vital Statistics Reports, January 2024
American College of Obstetricians and Gynecologists, Definition of Term Pregnancy, November 2013
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