Obstetric Ultrasonography in the Second Trimester

1

What are the indications for obstetric ultrasonography (US) in the second (and third) trimester?

There are numerous indications for US imaging in the second (and third) trimester, the most common ones including screening evaluation of fetal anatomy; screening for fetal anomalies; estimation of gestational age; and evaluation of fetal growth, fetal well-being, fetal presentation, cervical insufficiency, amniotic fluid status, placental location, and vaginal bleeding. In most circumstances, US in the second and third trimester only requires transabdominal imaging. Transvaginal or transperineal imaging may be required for more detailed evaluation of the cervix and placental position in cases with placenta previa, suboptimal visualization of the cervix, or with a shortened cervix on transabdominal imaging.

2

List the basic components of a standard (level 1) second trimester US examination.

A standard second trimester US examination includes documentation of the following:

• Fetal number and presentation.

• Fetal cardiac activity.

• Estimate of amniotic fluid volume.

• Location, appearance, and relationship of the placenta to the internal cervical os.

• Placental location.

• Gestational age.

• Fetal weight estimation.

• Fetal anatomic survey, including head (cerebral ventricles, choroid plexus, midline falx, cerebellum, cistern magna, nose and lips), chest (four-chamber heart, right and left ventricular outflow tracts), abdomen (situs, stomach, kidneys, urinary bladder, fetal umbilical cord insertion site, umbilical cord vessel number), spine (cervical, thoracic, lumbar, sacral), extremities (arms and legs), gender in multiple gestations.

• Maternal uterus, cervix, and adnexa.

3

How is gestational age determined in the second trimester?

The most accurate estimation of gestational age is in the first trimester based on embryonic crown-rump length. In the second trimester (after 14 weeks gestation), dating is based on a combination of fetal measurements including biparietal diameter (BPD) (measured in the transverse plane at the level of the thalami from the outer edge of the proximal skull to the inner edge of the distal skull), head circumference (HC) (also measured at the level of the thalami), abdominal circumference (AC) (measured at the level of the fetal stomach and junction of the umbilical vein and portal venous sinus), and femoral diaphysis length (FL) ( Figure 59-1 ). These measurements are also used in calculations for estimation of fetal weight. A head-to-abdominal circumference ratio can also be calculated to assess appropriate fetal growth. The HC is normally larger than the AC in the second and early third trimesters, with a reversal of this ratio at term.

4

How is the amniotic fluid volume measured?

Amniotic fluid volume can be assessed qualitatively (by an experienced imager) or measured semiquantitatively. For semiquantitative assessment, the amniotic fluid index (AFI) is calculated by individually measuring and then adding together the deepest vertical empty fluid pockets (no umbilical cord or fetal parts) in each quadrant. A normal AFI is between 8 cm and 24 cm, and each individual fluid pocket should be between 2 cm and 8 cm.

5

What is oligohydramnios?

Oligohydramnios is too little amniotic fluid, defined quantitatively as a single deepest vertical fluid pocket depth less than 2 cm or total AFI of less than 8 cm. A lack of amniotic fluid can impair fetal lung development and result in fetal pulmonary hypoplasia with high fetal morbidity and mortality. Oligohydramnios can result from a variety of etiologies including fetal d emise, fetal r enal abnormalities, i ntrauterine growth retardation (IUGR), p remature rupture of membranes, p ost dates, and c hromosomal abnormalities (mnemonic DRIPPC).

6

What is polyhydramnios?

Polyhydramnios is too much amniotic fluid, defined quantitatively as a single deepest vertical pocket measurement greater than 8 cm or total AFI of greater than 24 cm. Polyhydramnios may be idiopathic (in up to 40%), secondary to maternal factors such as diabetes and hypertension, or secondary to fetal factors including anything that might impair fetal swallowing of amniotic fluid (upper gastrointestinal tract obstruction, chest narrowing or mass, and severe central nervous system abnormalities) and fetal hydrops.

7

How is cervical incompetence evaluated sonographically?

Cervical length can first be assessed transabdominally with a partially distended bladder. A fully distended bladder may falsely lengthen the cervix due to mass effect. If the cervix is not well visualized or appears shortened transabdominally, then transvaginal or transperineal imaging should be performed. The normal cervical length is ≥3 cm. A length of 2 to 3 cm is considered borderline shortened, and <2 cm definitively shortened. If the internal os is open (referred to as funneling), the maximal width of the dilated cervical canal should also be measured, and any prolapse of membranes, umbilical cord, and/or fetal parts should be documented ( Figure 59-2 ). The major risk of an incompetent cervix is preterm delivery.

8

Describe the various types of placenta previa.

The placenta normally terminates >2 cm above the internal cervical os. Extension of the placental edge within 2 cm of the internal os is referred to as low-lying placenta. Extension of the placenta up to or over the cervix is referred to as placenta previa. Placenta previa can be marginal (at the edge of the internal os) ( Figure 59-3 ), partial (partial coverage of the internal os), or complete (complete coverage of the internal os). Some cases of placenta previa may resolve as the pregnancy progresses, and therefore follow-up imaging is performed in the third trimester. If a placenta previa persists, then plans can be made for a cesarean section.

9

Differentiate the various types of placenta accreta.

Invasion of the placenta into the uterine wall is referred to as placenta accreta. Risk factors for placenta accreta include prior placenta previa, prior uterine surgery (cesarean section, myomectomy), prior retained products of conception, and multiparity. Placenta accreta is further defined as accreta vera (also called accreta; the most common type) if the placenta is only adherent to the uterine myometrium, increta if the placenta invades into the myometrium, and percreta if the placenta completely penetrates the myometrium (usually then extending up to/into the bladder). All forms of placenta accreta can be difficult to diagnose sonographically, but suggestive US features include absence of the normal subplacental hypoechoic zone, heterogeneous placental echotexture, subplacental vascular spaces, and placental extension into the bladder wall. The major complication of placenta accreta is massive maternal hemorrhage at delivery.

10

Does US have a role in assessing second (and third) trimester maternal pain and bleeding?

The major consideration in any pregnant patient presenting with pelvic pain or bleeding is placental hemorrhage and placental abruption. Risk factors for placental hemorrhage include maternal hypertension, smoking, alcohol or cocaine use, trauma, and premature rupture of membranes. The role of US is to detect hemorrhage/hematoma associated with the placenta. Hemorrhage is describe as retroplacental if it is located between the placenta and uterine wall, marginal if it is located lateral to the placenta, and intraplacental if it is located within the placenta. Placental abruption occurs if a retroplacental hemorrhage is large enough to separate the placenta from the uterus ( Figure 59-4 ). Placental bleeds can be difficult to detect sonographically if the blood readily decompresses through the vagina rather than building up as a measurable hematoma. Visualization of a retroplacental hematoma on US is associated with a worse outcome for the fetus and mother due to a higher risk of complete placental separation. Depending on the age of the placental bleed, the blood products may range from hypoechoic or completely anechoic in the acute and chronic phases to hyperechoic and heterogeneous in the subacute phase.