A 38-year-old woman, gravida 4 para 3, at 18 weeks gestation comes to the office for a routine anatomy ultrasound. The patient has had no vaginal bleeding, leakage of fluid, or contractions. She has no chronic medical conditions and her 3 prior pregnancies resulted in uncomplicated cesarean deliveries. Blood pressure is 118/66 mm Hg and pulse is 94/min. Fundal height is appropriate for gestational age. Transabdominal ultrasound reveals a fetus measuring 18 weeks gestation, an amniotic fluid index of 8 cm (normal: >5 cm), and an anterior placenta that covers the internal cervical os and penetrates the uterine myometrium. This patient is at greatest risk of placental invasion into which of the following structures?
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This patient has placental accreta spectrum, a disorder defined by the depth of abnormal placental invasion: placenta accreta (attachment to the myometrium), placenta increta (invasion into the myometrium), and placenta percreta (invasion through the myometrium and uterine serosa and into extrauterine structures).
The most common site for abnormal placental implantation is over a prior uterine scar; these areas have thin myometrium and lack the decidua basalis layer needed to form a normal uteroplacental interface. Patients with repeat cesarean deliveries commonly have an anterior, low transverse scar; therefore, placental implantation in future pregnancies is typically anterior and low, which can result in the placenta covering the internal cervical os (ie, placenta previa). Eventually, an invasive anterior placenta can separate the uterine scar and invade across the uterine wall into adjacent structures (ie, placenta percreta). The structure immediately anterior to the uterus is the bladder, so patients with an anterior placenta percreta are at greatest risk for bladder invasion.
(Choice B) The broad ligament is a wide fold of peritoneum containing the round ligament, the uteroovarian ligament, and the fallopian tube. Because it extends laterally from the uterus to the pelvic sidewall, it is at lower risk for invasion by an anterior placenta.
(Choice C) Although placentas can implant over the internal cervical os, placental invasion of the cervix does not usually occur because the cervix is composed of dense connective tissue and is much firmer, less vascular, and less vulnerable to penetration compared to the myometrium.
(Choice D) The infundibulopelvic ligament (ie, suspensory ligament of the ovary), which contains the ovarian vessels, connects the ovary to the pelvic sidewall. Because it is lateral to the uterus, it is unlikely to be affected by placental invasion.
(Choice E) The uterosacral ligament anchors the posterior uterus to the sacrum for pelvic support. Because it attaches to the posterior uterus, it is unlikely to be invaded by an anterior placenta.
(Choice F) The ureter travels along the lateral pelvic sidewall before diving posterior to the uterine vessels and entering the bladder. Its path is posterior and lateral to the uterus, making placental invasion by an anterior placenta unlikely.
Educational objective:
Placenta percreta is placental invasion through the myometrium and uterine serosa and into extrauterine structures. An anterior placenta percreta is most likely to invade structures anterior to the uterus such as the bladder.