Late-term and postterm pregnancy: Clinical sciences

Late-term and postterm pregnancies occur when a pregnancy continues beyond the standardized 40 weeks of gestation. More specifically, late-term refers to those pregnancies at or beyond 41 weeks of gestation and postterm refers to pregnancies at or beyond 42 weeks of gestation. Both are associated with increased risks of maternal and fetal morbidity and mortality. Intrapartum risks are related to an increased incidence of oligohydramnios, which is a decreased amniotic fluid that’s associated with umbilical cord compression, fetal heart rate abnormalities, and meconium-stained fluid.

When it comes to delivery risks, there is an increased risk of operative-assisted vaginal deliveries or c-section deliveries, shoulder dystocia, severe perineal lacerations, and postpartum hemorrhage. Now, there are also some neonatal risks as well, such as meconium aspiration syndrome, neonatal convulsions, 5-minute Apgar scores of less than 4, and increased rates of NICU admissions.

Finally, while the absolute risk is low, the risk of stillbirth increases with each subsequent week of gestation after week 40. Thus, it is important to accurately identify late-term and postterm pregnancies, categorize pregnancies into high and low-risk states, and initiate appropriate antenatal testing once a patient enters this stage of gestation.

When a patient presents with a late-term or postterm pregnancy, you should start with a focused history and physical exam. History might reveal some associated risk factors for late-term and postterm pregnancy including nulliparity, a history of a prior late-term or postterm pregnancy, carrying a male fetus, and certain fetal disorders in the current pregnancy, such as anencephaly. On physical exam, you may note a BMI of 30 or more, as obesity is a risk factor for prolonged pregnancy. Additionally, the dating of the pregnancy will be confirmed to be at least 39 and 0/7 weeks of gestation.

Now that you have obtained some helpful information, your next step is to determine if your patient has a high or low-risk pregnancy. This is important because patients with high-risk pregnancies require additional antenatal monitoring. Also, they will likely need to be delivered by their estimated due date, or by 40 weeks of gestation, to decrease associated maternal, fetal, and neonatal risks.

Some factors that classify a pregnancy as high risk include advanced maternal age, or AMA; pregnancy as a result of in-vitro fertilization or IVF; fetal disorders, such as anencephaly; hypertensive disorders of pregnancy like preeclampsia; gestational diabetes; intrauterine growth restriction or IUGR; maternal illness such as lupus, and renal or thyroid disease; a history of prior stillbirth; and placental disorders such as polyhydramnios, oligohydramnios, or a single umbilical artery.

Okay, if your patient has a high-risk pregnancy, initiate regular third trimester antenatal testing, which is performed to assess fetal well-being. Antenatal testing may include non-stress tests, or NSTs; assessment of amniotic fluid index, also known as AFI; biophysical profiles, or BPPs; and growth ultrasounds. The frequency of antenatal testing varies for each patient but ranges from once per week to multiple times each week.

Now, non-reassuring testing may include non-reactive NST or a fetal heart tracing with decelerations on an NST; polyhydramnios or oligohydramnios on AFI measurement; a low BPP score; or inadequate fetal growth on ultrasound. If at any point after 39 weeks of gestation the testing is non-reassuring, you should proceed with delivery.

Delivery is completed either by induction of labor or c-section for those with contraindications to vaginal delivery. On the flip side, reassuring antenatal testing will show reactive NSTs with moderate variability and accelerations; normal AFI measurement; a reassuring BPP and adequate fetal growth on ultrasound. In this situation, as you are dealing with a term pregnancy, you can allow it to continue beyond 39 weeks. For these patients, you should plan for delivery by 40 weeks and 0 days to minimize the increased risk of morbidity to both the mother and the baby. Again, this can be either by induction of labor or scheduled c-section.