19.1 Placenta: Normal, Previa, Abruption & Accreta

Normal Placental Development and Documentation

The placenta develops from the chorion frondosum, the portion of the chorionic sac with abundant villi that burrows into the decidua basalis at the implantation site. The villi on the opposite (decidua capsularis) side atrophy to form the smooth chorion laeve. By the end of the first trimester the discoid placenta is identifiable sonographically as a homogeneous, moderately echogenic structure along the uterine wall. At term it normally measures roughly 15-20 cm in diameter and 2-4 cm in thickness; a thickness greater than about 4-5 cm defines placentomegaly, seen with maternal diabetes, hydrops, and congenital infection.

Every obstetric scan must document placental location (anterior, posterior, fundal, or lateral) and, most importantly, its relationship to the internal os of the cervix. Transabdominal scanning with a full bladder can create a false-positive impression of previa through lower-segment compression, so transvaginal or translabial imaging is the standard for measuring the exact placental edge-to-os distance.

Grannum Placental Grading

Grade III appearing before 36 weeks is abnormal and suggests placental insufficiency — premature maturation with calcifications and a small placenta, strongly linked to intrauterine growth restriction (IUGR) and oligohydramnios.

Placenta Previa

Previa terminology was simplified by the 2013 Fetal Imaging Workshop (SMFM, AIUM, ACR, ACOG), which retired the terms partial previa and marginal previa. Two terms now describe the placental edge relative to the internal os:

• Placenta previa — the placenta covers or reaches the internal os. (The legacy grades complete/total, partial, and marginal all collapse into this single term; expect the old vocabulary on older question banks but report the current one.)
• Low-lying placenta — the edge lies within 2 cm of the os but does not reach it
• Normal — the placental edge is more than 2 cm from the os

Because the lower uterine segment expands as pregnancy advances, many second-trimester "previas" resolve by placental migration (actually differential uterine growth). Standard practice is to re-scan at approximately 32 weeks before confirming the diagnosis. A persistent complete previa mandates cesarean delivery and raises concern for vasa previa when the placenta is low-lying or the cord insertion is velamentous.

Placental Abruption

Abruption is premature separation of a normally implanted placenta, producing a retroplacental hematoma. Bleeding may be external (revealed) — bright vaginal bleeding, classically painful — or concealed (roughly 20%), in which blood is trapped behind the placenta and the uterus becomes rigid and hypertonic without visible hemorrhage. Sonographically a fresh hematoma may be isoechoic to the placenta and easily missed; it becomes progressively hypoechoic over 1-2 weeks. Hematomas are described as retroplacental (behind the placenta, worst prognosis), marginal (subchorionic, at the placental edge, best prognosis), or preplacental (subamniotic). Sensitivity of ultrasound for abruption is limited, so a negative scan never excludes it.

Placenta Accreta Spectrum

In placenta accreta spectrum, villi attach abnormally because of a defective decidua basalis (Nitabuch layer):

• Accreta (~75-78%) — villi adhere to the myometrium
• Increta (~17%) — villi invade into the myometrium
• Percreta (~5%) — villi penetrate through the myometrium to or beyond the serosa, sometimes into bladder

Key sonographic signs include loss of the normal retroplacental clear space, numerous irregular placental lacunae ("moth-eaten" appearance) with turbulent color Doppler flow, and myometrial thinning to less than 1 mm over the placenta. The dominant risk factor combination is prior cesarean delivery plus placenta previa — risk rises steeply with each successive cesarean.

Abnormal Cord Insertions and Lobation

A succenturiate lobe is an accessory placental lobe connected to the main disc by fetal vessels; it risks retained placenta and postpartum hemorrhage, and the unprotected connecting vessels create vasa previa risk. Battledore (marginal) insertion is cord attachment at the placental edge; velamentous insertion is attachment to the membranes, leaving vessels unsupported by Wharton jelly.

Gestational Trophoblastic Disease

A complete mole is diploid (46,XX, entirely paternal origin) with no fetus, a vesicular "snowstorm" intrauterine mass, markedly elevated human chorionic gonadotropin (hCG), and bilateral theca lutein cysts; about 15-20% progress to malignant disease. A partial mole is triploid (69,XXY) with an abnormal fetus and focal vesicular change. An invasive mole penetrates the myometrium and may appear as an echogenic myometrial mass with rich vascularity.

Additional clinical pearls sharpen the differential. The classic presentation of placenta previa is painless, bright-red vaginal bleeding in the late second or third trimester, and digital cervical examination is strictly contraindicated until previa is excluded, because it can trigger catastrophic hemorrhage. This contrasts with abruption, which classically causes painful bleeding with uterine tenderness and hypertonus; risk factors include hypertension, preeclampsia, trauma, cocaine use, smoking, and prior abruption. Prognosis in abruption tracks the size of the retroplacental collection: hematomas involving more than about 50% of the placental surface carry high rates of fetal demise, while small marginal subchorionic collections often resolve. For accreta spectrum, the quantitative risk is worth memorizing: with placenta previa, the probability of accreta rises from roughly 3% after one cesarean to about 11% after two, 40% after three, and over 60% after four or more. After evacuation of a molar pregnancy, serial hCG levels are followed to detect persistent trophoblastic disease; an invasive mole invades the myometrium but retains chorionic villi, whereas choriocarcinoma — the most malignant form — contains no villi and metastasizes hematogenously to lung, liver, and brain.