The American College of Obstetricians and Gynecologists (ACOG) is the organization that US doctors turn to for practice recommendations. This post will review their guidelines on the management of late-term and postterm pregnancies. If you go past your due date, this is the information your doctor will be using to guide your care. Should babies just “come when they’re ready”? What are the risks?
American College of Obstetrics and Gynecology. (2014). ACOG practice bulletin no. 146: Management of late-term and postterm pregnancies.
Pregnancy is considered full term between 37 0/7 weeks and 41 6/7 weeks (the due date is at 40 0/7 weeks). A pregnancy between 41 0/7 weeks and 41 6/7 weeks gestation is considered a “late-term pregnancy”, while one that is beyond 42 0/7 weeks is considered “postterm”. Factors that increase the likelihood of postterm pregnancy include it being a first pregnancy, having a history of a previous postterm pregnancy, having a male fetus, maternal obesity, genetic predisposition, certain fetal disorders (such as anencephaly), or incorrect dating.
Fetal and Neonatal Risks
Studies indicate that late-term and postterm pregnancies are associated with increased fetal morbidity and mortality (harm or death). These include, “increased risk of neonatal convulsions, meconium aspiration syndrome, and 5-minute APGAR scores of less than 4.” Babies born after 42 weeks were also significantly more likely to be admitted to the neonatal intensive care unit.
While most late- and postterm babies are an appropriate weight, they are at two times greater risk of macrosomia (a birth weight of over 8lbs 13oz, or 4,000g). This is thought to contribute to the increased rate of operative vaginal delivery (vacuum or forceps), cesarean sections, and shoulder dystocias that are found in postterm pregnancies.
“Postmaturity syndrome” is noted in 10-20% of postterm pregnancies. Signs include decreased subcutaneous fat, vernix and lanugo. Meconium staining is also frequently seen.
Oligohydramnios is found more often in pregnancies after 42 0/7 weeks. These pregnancies have an increased risk of abnormal fetal heart rates, umbilical cord compression, meconium stained fluid, and low APGAR scores.
While the absolute risk of stillbirth in postterm pregnancy is low, research does show an increased risk. One study showed, “an eightfold higher rate of stillbirth at 43 weeks that at 37 weeks… [Another study] showed a similar significant increase in the risk of stillbirth from 37 weeks (0.4/1,000) to 43 weeks (11.5/1,000).
There are also maternal risks in a postterm pregnancy. One large study showed that the risk of, “severe perineal laceration, infection, postpartum hemorrhage, and cesarean delivery were all increased in late-term and postterm pregnancies.” However expectant management until reaching the postterm period is recommended in low risk pregnancies.
Are there interventions that decrease the incidence of late-term and postterm pregnancies?
Having an accurate due date and gestational age are the best ways to decrease the diagnosis of late-term and postterm pregnancy. Using only last menstrual period to determine the due date has been shown by studies to be unreliable, leading to increased likelihood of postterm pregnancy. This is because of inaccurate maternal recall, as well as differences in timing of ovulation. Early ultrasound in the first trimester is the most accurate method to determine or confirm the due date. Using ultrasound decreased the rate of postterm pregnancies from 9.5% to 1.5%.
Membrane sweeping has also been shown to decrease the risk of late-term and postterm pregnancies. A Cochrane review showed that, “membrane sweeping was associated with a significant reduction in the number of pregnancies that progressed beyond 41 weeks gestation.” Possible side effects can include vaginal bleeding and maternal discomfort. Women with placenta previa, or other conditions that make labor or vaginal delivery potentially unsafe, should not have their membranes stripped.
Should antepartum fetal testing be performed in late-term and postterm pregnancies?
No randomized controlled trials have been done to demonstrate that testing decreased rates of perinatal morbidity or mortality in late-term and postterm pregnancies. However, because of the data showing increased risk of stillbirth at or beyond 41 0/7 weeks, initiation of fetal testing after this point may be indicated.
What type of antepartum fetal surveillance should be used, and how frequently should testing be performed in late-term and postterm pregnancies?
While antepartum fetal testing may be indicated in pregnancies after 41 0/7 weeks, there is not enough data to show which method should be preferred or the optimal frequency of testing. A number of small studies suggest that twice weekly testing is superior to once weekly testing in postterm pregnancies. Pregnancies after 41 0/7 weeks are at increased risk of oligohydramnios, and it is recommended that ultrasound be used to assess amniotic fluid levels. Traditionally, oligohydramnios has been defined as a fluid level of 5cm or less. However, research is showing that defining oligohydramnios as a deepest vertical pocket of 2cm or less reduces unnecessary interventions without increasing adverse outcomes. Pregnancies complicated with oligohydramnios are more likely to have meconium fluid, increased risk of fetal demise, fetal growth restriction, and fetal heart rate abnormalities. If oligohydramnios is found at 41 0/7 weeks or greater, induction is warranted.
When should labor be induced in the late-term or postterm pregnancy?
One of the largest studies available compared women at 41 0/7 weeks who were either induced, or received expectant management with fetal surveillance two to three times weekly. They found, “an increased rate of cesarean deliveries in the expectantly managed group, although there were no differences in the rates of perinatal mortality and neonatal morbidity.” A cochrane review showed that induction after 41 0/7 weeks was associated with a decreased risk of perinatal death, cesarean delivery, and meconium aspiration syndrome. The conclude that, “induction of labor between 41 0/7 weeks and 42 0/7 weeks can be considered. Induction of labor after 42 0/7 weeks and by 42 6/7 weeks of gestation is recommended given evidence of an increase in perinatal morbidity and mortality.”
Is there a role for vaginal birth after cesarean delivery in the management of postterm pregnancy?
Successful vaginal birth after cesarean (VBAC) is associated with lower rates of maternal and neonatal complications, and a trial of labor is a reasonable option in postterm pregnancies. One large study showed no increase in the uterine rupture rate at or beyond the estimated date of delivery. However, the likelihood of VBAC success decreased with increasing gestational age. The failure rate is 22.2% at 40 weeks, increasing to 35.4% after 41 weeks. “Awaiting spontaneous labor, as opposed to undergoing labor induction, most likely avoids further additional increased risk of uterine rupture. Thus, TOLAC (trial of labor after cesarean) remains an option for women with postterm pregnancies… but these women should be counseled regarding their individual risks such as failure of TOLAC and of uterine rupture.”
Going past your due date is a “hot button” issue for people in the birthing world. Many doctors look at the research and see the increasing risk of stillborn with advancing gestational age and think, “If 42 weeks is safe, 41 is safer!” They feel the risks of stillbirth outweigh the risks of induction, and recommend delivery at 41, or sometimes even 40 weeks. They don’t take into account that with accurate dating, 98.5% of women will go into labor on their own prior to 42 weeks. Even without completely accurate dating using only last menstrual period, 90.5% of women will go into labor before induction for postterm pregnancy is recommended. Most of those inductions would have been completely unnecessary if some patience and expectant management were used. Even without an ultrasound confirmed due date, ACOG does not recommend induction before 42 1/7 weeks. Between 41 and 42 weeks, they only say it may be considered. It’s certainly not set in stone. If your pregnancy is low risk, patience is key.
On the other side of the coin are the people who say, “due dates are arbitrary, babies will come when they are ready!”. They feel the risk of inducing a baby who may not be ready yet outweighs the risk of stillbirth by going postterm. Babies who are induced without a medical reason before they are ready are more likely to have breathing and feeding difficulties, and be admitted to the NICU, because they are not as developed as they would have been if labor would have started on its own time. There are lots of anecdotal stories of women going past 42 weeks, even 43 weeks, without issue. Most of these women, however, fall into the 8% of postterm pregnancies that aren’t really postterm- they’re just inaccurately dated and didn’t know it. Because they were inaccurately dated, the risks of going postterm didn’t apply to them. In these cases, the “it’s too risky to induce” people are absolutely right. Induction carries it’s own risks, and increases the pain of contractions. That “41 week late-term induction” may really be a 39 week “elective” induction.
However, that ignores the 1.5% of pregnancies that are actually postterm, and that do have risk. While the absolute risk is small (you’re far, far more likely to have a healthy baby than not, even if you do go past your due date), the risk is still there and shouldn’t be completely brushed off.
As far as determining your due date, if you are a strict cycle and fertility tracker with regular cycles, you may feel comfortable using this to determine your due date (although research shows that many “known” last menstrual periods may still not accurately reflect gestational age due to differences such as ovulation time). However, if you are unsure about the exact day, or have irregular cycles or ones that are shorter or longer than average, you may want to consider early ultrasound dating. This can become especially important if your midwife has to risk you out of her practice after 42 0/7 weeks. If you are one of the 8% that would have been risked out for postterm pregnancy, the ultrasound might buy you a week or two more in her care.
For more information on research regarding induction for postterm pregnancies, check out Evidence Based Birth’s article at: http://evidencebasedbirth.com/evidence-on-inducing-labor-for-going-past-your-due-date/