Prolonged Pregnancy

Hello. I’m Juan Sebastian Sandoval, one of the OBGYN residents at Duke University. And today we’re going to talk about prolonged pregnancy. Given that this is a review of the USMLE, we will pay particular attention to the key information that you need to know and not the details that, although are important for the practice of an obstetrician, are not relevant for these examinations. During this presentation, we are going to define prolonged pregnancy, recognize the etiology and risk factors, understand the consequences of this condition, and identify management options. A 25-year-old G2P1001 presents to your office to establish prenatal care. She recently immigrated to the United States, hence you don’t have any of her medical records. And she states she is currently at 42 weeks and 5 days of gestational age. According to her, this is based on her last menstrual period, of which she is certain because ever since her menarche she annotates in a calendar the first day of her menstruation. On today’s ultrasound, the fetus has an estimated gestational age of 39 weeks. On examination, you find her cervix to be 4 centimeters dilated, 60% effaced, anterior, and soft. The fetus is cephalic. What is the next step in the management of this patient? Prolonged pregnancy is defined as a gestational equal or greater to 294 days, which is equivalent to 42 weeks after the first day of the last menstrual period. Although up until now there is no agreement on the correct term for this condition, ACOG states that we should use “prolonged pregnancy” to describe a pregnancy that has lasted more than the upper limit of a normal-term gestational. The 42-week mark is somewhat arbitrary as it was established before the widespread use of antenatal testing. In fact, recent data shows that there is a marked increase in the prenatal morbidity and mortality after 41 weeks of gestation. Anyway, whether it’s called post dates, post terms, post maturity, or prolonged pregnancy, it is not relevant for the purpose of the USMLE. What you really need to know is that this is a high-risk condition as it puts the fetus at an increased risk of poor prenatal outcomes. So is it common? Yes. 7% to 12% of pregnancies are complicated by this condition. When it comes to the etiology, it’s key to know that the most common cause of prolonged pregnancy is an error in determining the estimated delivery date or gestational age. This is a consequence of variations in the menstrual cycle length and inaccuracy in remembering the date of the last menstrual period. Conversely, the most common cause of true prolonged pregnancy is idiopathic, apparently due to a biological variability of the duration of pregnancy. So who’s at risk? Although the exact triggers for initiation of labor have not been completely described yet, there are certain fetal and placental abnormalities– such as anencephaly, placental sulfatase deficiency, and fetal adrenal hpyoplasia– that have been associated with increased risk of prolonged pregnancy. Since there is up to a 50% recurrence risk in mothers who previously had a prolonged pregnancy, the genetic predisposition for this condition is evident. Among the most important things you have to know about the entire presentation is that the consequence of prolonged pregnancy is an increased risk in prenatal morbidity and mortality. Data shows that this effect is even greater in fetuses small for gestational age when compared to the ones adequate for gestational age. The main determinant of the type of morbidity and mortality associated with prolonged pregnancy is the placental function. The fetus relies 100% on the placenta for its nutrition. If it continues to work appropriately, he will continue to grow, being adequate for gestational age but having a high risk of macrosomy. Conversely, if the placental function is inadequate, the fetus will be literally starving, which increases its risk for being small for gestational age and having dysmaturity syndrome. 80% of babies born after a prolonged pregnancy will continue to have adequate placental function, making them macrosomic. The main risk in this group of patients is labor dysfunction, which increases occurrence of shoulder dystocia and its complications; cesarean section; postpartum hemorrhage; operative vaginal deliveries– like forceps-assisted vaginal delivery and vacuum-assisted vaginal delivery; and birth trauma in general. 20% of babies born after a prolonged pregnancy will have inadequate placental function, leading them to suffer from dysmaturity syndrome. These neonates are at a higher risk of being small for gestational age, having fetal growth restriction, oligohydramnios, fetal distress, meconium-stained fluid, acidosis, and needing a cesarean section. So, what will you do if your patient suffers from this condition? The first step is to determine if there is good dating. So let’s assume there’s poor dating. The management should be expectant. Remember that the further the gestational age, the less reliable an ultrasound is for estimating it. In this case, you have to perform twice-weekly fetal well-being testing with a biophysical profile and non-stress tests. If there’s good dating– meaning there’s a reliable LMP and/or early first trimester ultrasound– the next step is to evaluate the cervix. If the cervix is favorable, there’s no benefit of keeping the fetus in utero. You have to manage the patient actively to promote labor. A first step could be sweeping the amniotic membranes, which can trigger labor by the release of factors such as prostaglandins. Another option is to induce labor with any of the multiple mechanical or pharmaceutical methods of labor induction– in example, misoprostol, pitocin, laminaria, or cervical ripening balloons. Now, if the cervix isn’t favorable, the management is controversial. There are multiple factors that have to be put into consideration in order to counsel the patient about management options. Luckily for you, it’s not typical for USMLE to include questions about controversial subjects where there is no consensus of only one clear best answer. Having said all of this, the key elements that you need to know back and forth about prolonged pregnancy are, first, it’s defined as a pregnancy that has reached or exceeded 42 weeks of gestational age. Second, the most common cause of prolonged pregnancy is poor dating. Third, morbidity and mortality depend on placental function, which, if it’s decreased, the consequence is dysmaturity syndrome and, if it’s maintained, macrosomnia syndrome. Fourth and finally, the management depends on the dating and the cervix. If poor dating, it should be expectant. If good dating, the next step is to check the cervix. If it’s favorable, you should proceed with induction of labor. If unfavorable, the management is controversial.

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