Abnormal Placentation after Cesarean Delivery

Erin H. Burnett

INTRODUCTION AND STATISTICS

Cesareans continue to be the most commonly performed procedure in the United States accounting for about 32% of the births nationwide. These critical issues of abnormal placentation will likely become more prevalent. Thankfully ultrasound technology has vastly improved making diagnoses easier, but facilities must still continue to train ultrasound teams to carefully watch for these abnormal placentations, especially in the setting of previous cesareans or other uterine surgeries. This chapter will discuss the various types of abnormal placentation including low-lying placenta, placenta previa, placenta accreta spectrum (PAS) which refers to the creta spectrum (accreta, increta, and percreta), and cesarean scar pregnancy (CSP).

In 2015 and 2016, nearly 4 million births per year were registered in the United States.¹ This number has declined slightly since the most recent peak in 2007 when over 4.3 million births were registered. The cesarean delivery rate has been relatively stable around 32% for the past 8 years.¹ Before 2009, the cesarean delivery rate had steadily increased every year since 1996 when it was 20.7%.¹ The “low-risk” cesarean rate has been steady at 25% to 27% in the past 7 years.¹ “Low risk” implies a singleton, term (37 weeks gestation or beyond), and cephalic presentation having a cesarean for their first pregnancy.¹ The VBAC (vaginal birth after cesarean) rate in 2016 was only 12.4% (Table 4-1).¹

RISK FACTORS ASSOCIATED WITH CESAREAN MORBIDITY

As cesareans rates rise over the years, so too has the morbidity associated with them. The majority of uterine scars heal normally; however, some can be found to have an anterior uterine wall deficiency.² Increasing number of prior cesarean increases the surface area of the scar and the risk of fibrosis, which has known poor vascularity, and hence raises the risk of inappropriate healing.²,³ Hence, the uterine scar left behind after a cesarean increases the risk of PAS, scar dehiscence, and a rare form of ectopic pregnancy know as a cesarean scar pregnancy (CSP).⁴ Studies estimate that of the cases where the placenta implants on the cesarean scar, 30% to 40% have abnormal placentation.⁵,⁶ This exponentially increases the risk for hemorrhage and hysterectomy.

TABLE 4-1 Births by Method of Delivery and Race and Hispanic Origin of Mother in the United States (2010-2016)

Year and Race and Hispanic Origin	All Births	Vaginal		Cesarean			Not Stated	Cesarean			Vaginal Birth After Previous Cesarean Delivery^g
Year and Race and Hispanic Origin	All Births	Total^a	After Previous Cesarean	Total^b	Primary	Low-risk^c	Not Stated	Total^d	Primary^e	Low-risk^f	Vaginal Birth After Previous Cesarean Delivery^g
All Races and Origins^h	Number							Percent
2016	3,945,875	2,684,803	75,244	1,258,581	728,500	329,614	2491	31.9	21.8	25.7	12.4
2015	3,978,497	2,703,504	^—	1,272,503	^—	331,982	2490	32.0	^—	25.8	^—
2014	3,988,076	2,699,951	^—	1,284,551	^—	337,086	3574	32.2	^—	26.0	^—
2013	3,932,181	2,642,892	^—	1,284,339	^—	344,405	4950	32.7	^—	26.8	^—
2012	3,952,841	2,650,744	^—	1,296,070	^—	355,942	6027	32.8	^—	27.2	^—
2011	3,953,590	2,651,428	^—	1,293,267	^—	359,669	8895	32.8	^—	27.2	^—
2010	3,999,386	2,680,947	^—	1,309,182	^—	368,523	9257	32.8	^—	27.5	^—
Non-hispanic, Single Race (2016)ⁱ:
White	2,056,332	1,419,788	37,442	635,588	379,240	172,006	956	30.9	21.5	24.7	12.8
Black	558,622	357,859	11,763	200,460	117,410	50,287	303	35.9	25.4	30.3	12.4
Hispanic^j	918,447	627,095	17,847	290,832	153,462	67,278	520	31.7	20.1	25.1	11.5
^—Comparable data not available for the 50 states and District of Columbia for 2010 to 2015 because not all reporting areas had adopted the 2003 US Standard Certificate of Live Birth.^aIncludes unknown type of vaginal delivery; see Technical Notes.^bIncludes unknown type of cesarean delivery; see Technical Notes.^cLow-risk cesarean is defined as singleton, term (37 completed weeks or more of gestation based on the obstetric estimate), cephalic, cesarean deliveries to women having a first birth.^dPercentage of all live births delivered by cesarean.^ePrimary cesarean rate is the number of births to women having a cesarean delivery per 100 births to women without a previous cesarean.^fLow-cesarean rate is the number of singleton, term (37 wk or more of gestation based on the obstetric estimate), cephalic, cesarean deliveries to women having a first birth per 100 women delivering singleton, term, cephalic, first births.^gVaginal birth after cesarean delivery rate is the number of births to women having a vaginal delivery per 100 births to women with a previous cesarean delivery.^hIncludes births to race and origin groups not shown separately, such as Hispanic single-race white, Hispanic single-race black, and non-Hispanic multiple-race women, as well as births with origin not stated.ⁱRace and Hispanic origin are reported separately on birth certificates; persons of Hispanic origin may be of any race. In this table, non-women are classified by race. Race categories are consistent with 1997 Office of Management and Budget standards; see Technical Notes. Single race is defined as only one race reported on the birth certificate.^jIncludes all persons of Hispanic origin of any race; see Technical Notes.
Reprinted with permission from Centers for Disease Control and Prevention. National Vital Statistics System, Natality. https://www.cdc.gov/nchs/nvss/index.htm.

Other uterine surgeries, besides cesareans, also contribute to the problem and include uterine curettage, hysteroscopy, myomectomy, endometrial ablation, and uterine artery embolization.⁴,⁷ All of these surgeries cause damage to or a deficiency of decidualized tissue and an increase in fibrosis at the scar site, hence increasing the risk of abnormal placentation.⁴,⁷ In addition, prior pelvic or whole body radiation or chemo therapy all have an increase incidence of accreta pointing to the theory of damaged decidua in these cases as well.⁸,⁹,¹⁰ However, defective decidualized tissue cannot be the only cause as many pathology specimens are found to have normal decidualization.¹¹,¹²

Multiple prior surgeries also compromise normal healing and lead to increased surface area of damaged tissue thus increasing the risk for abnormal placentation in subsequent pregnancies.¹³,¹⁴,¹⁵ Just like any wound, repeated trauma leads to a disruption in the healing process. Cesarean scar healing can be compared to skin healing where the highly vascular granulation tissue is replaced by avascular scar tissue.¹⁶ In patients with previous cesareans, the risk of abnormal placentation is fourfold higher if a central or anterior placenta previa is identified compared to a posterior previa.¹⁷

Downes identified a prelabor cesarean delivery, versus an intrapartum cesarean as a risk factor for previa, quoting a 2.62 odds ratio.¹⁸ In addition, surgical techniques used for closure may affect one’s risk. Closing the hysterotomy with a single layer versus a double layer results in a noninverting suture and may lead to deficient postoperative healing and therefore result in scar defects.¹⁴

IMPLANTATION SITES

Placenta location is typically determined for the first time during the anatomy scan around 18 to 20 weeks, unless a patient presents earlier for vaginal bleeding or other indications. The physical location of placental implantation should be described in one of three ways: previa, low-lying, and normal (usually anterior or posterior). In the past, during the preultrasound times, terms such as marginal, incomplete, and partial were used.¹⁹ These terms are very ambiguous and subjective, hence consistent classification was lacking. In addition, the management did not change between these various “types.”

TRANSVAGINAL ULTRASOUND AND IDENTIFICATION OF INTERNAL CERVICAL OS

Now that ultrasound is readily available, the location of the placenta can be determined by a transvaginal ultrasound (Figure 4-1A-D). After identifying the internal cervical os, the linear distance from the os to the edge of the placenta should be determined.¹⁹

Previa: placenta covers the os to any degree
Low-lying: the placenta does not cover the os, but lies within 2 cm of the os
Normal: implies the placenta is greater than 2 cm away from the cervical os

Location should be determined at the time of the anatomy scan (18-24 weeks).¹⁹ If there is concern via transabdominal ultrasound for a previa or low-lying placenta, then a transvaginal ultrasound should be performed for confirmation. For those patients who have a low-lying placenta or a placenta previa, the most common management is to reimage the location in the mid-third trimester. Some authors suggest that a low-lying of at least 1 cm away does not need to be reimaged as the risk of complications is low.¹⁹ Patients should be reassured that many abnormal placentations will resolve by the time of delivery.¹⁹ The lower uterine segment grows and develops, and the overall blood supply in this area is not ideal for placental growth, therefore as the placenta grows with gestation, it is more apt to grow toward the fundus and away from the lower uterine segment as it seeks better blood supply.¹⁹ Some believe that atrophy of the cells over the os occurs and therefore contributes to the resolution of previas (Table 4-2).¹⁹

FIGURE 4-1 A-D, Depict various placenta locations. A, Placenta previa, B and C, low-lying placentas, D, normal location. Red/horizontal arrow shows the internal cervical os. Blue/vertical arrow points to edge of the placenta. The linear distance measured is reported (yellow line).

MANAGEMENT DURING PREGNANCY

Women with placenta previa or vaginal bleeding in pregnancy should partake in pelvic rest; however, there is insufficient evidence to support bed rest.¹⁹

As previously discussed, most placentas initially implanted over or near the cervix will move with time and careful follow-up is generally acceptable. However, patients experiencing signs of preterm labor and or vaginal bleeding should seek care immediately.

Typical management of various placenta implantation sites can be seen in Figure 4-2.

Although the majority of patients with a low-lying placenta located 1 to 10 mm from the os result in cesareans, a patient should be counseled that vaginal deliveries are potentially still an option however there are increased risks (Table 4-3). These risks should be included in the counseling that occurs ideally before the onset of labor.

TABLE 4-2 Persistence of Placenta Previa according to Gestational Age at Ultrasound Detection

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