he timespan that deines a mid trimester fetal loss extends from the end of the first trimester until the fetus weighs < 500 g or gestational age reaches 20 weeks. Less than in the irst trimester,
TABLE 18-6. Some Causes of Midtrimester Spontaneous
Abruption, previa the spontaneous loss rate in the second ranges from 1.5 to 3 percent and, after 16 weeks, is only 1 percent (Simpson, 2007; Wyatt, 2005). Unlike earlier miscarriages that frequently are caused by chromosomal aneuploidies, these later fetal losses are due to a multitude of causes (Table 18-6). One frequently overlooked factor is that many second-trimester abortions are medically induced because of fetal abnormalities detected by prenatal screening programs for chromosome aneuploidy and structural defects.
.Midtrimester abortions are classified similarly to first-trimester miscarriages (p. 348). Management is similar in many regards to that used for second-trimester induced abortion, described later (p. 362). One exception is cervical cerclage, which may be employed for cervical insuiciency.
Also known as incompetent cervix, this is a discrete obstetrical entity characterized classically by painless cervical dilatation in the second trimester. It can be followed by prolapse and ballooning of membranes into the vagina, and ultimately, expulsion of an immature fetus. his sequence oten repeats in uture pregnancies.
Although the cause of insuiciency is obscure, previous cervical trauma has been implicated. One Norwegian cohort study of more than 15,000 women with prior cervical conization found a fourfold risk of pregnancy loss before 24 weeks' gestation (lbrechtsen, 2008). However, cerclage is not beneicial for women solely with this risk and without a preterm birth history (Zeisler, 199 ). Of other surgeries, dilation and evacuation carries a 5-percent risk of cervical injury, but neither it nor dilation and extraction raises the likelihood of cervical insuiciency (Chasen, 2005). In other instances, abnormal cervical development, including that following in utero exposure to diethylstilbestrol (DES), may playia role (Hoover, 2011).
Last, cervical ripening changes, such as altered hyaluronan or collagen content, discussed in Chapter 21 (p. 409), may be contributory (Eglinton, 2011; Sundtoft, 2017).
For women with an unequivocal history of second-trimester painless delivery, prophylactic cerclage placement is an option and reinforces a weak cervix by an encircling suture. However, some women have a history and clinical findings that make it diicult to veriy-classic cervical insuiciency. In one randomized trial of almost 1300 women with an atypical history, cerclage was found to be only marginally beneicial-13 versus 17 percent-to prolong pregnancy past 33 weeks (MacNaughton, 1993). It seems likely that many of these women instead had preterm labor.
In addition to history, the physical inding of early dilation of the internal cervical os may be an indicator of insuiciency. In a systematic review, cerclages that were placed based on this finding provided superior perinatal outcomes compared with expectant management (Ehsanipoor, 2015).