Obstetric Imaging

Reference

All ages in this section refer to the menstrual age or gestational age (GA) based on the last menstrual period (LMP) and not the embryonic age based on day of conception. A 4-week pregnancy by the LMP method thus corresponds to a 2-week pregnancy by the conception method. All measurements given in this section are for transvaginal sonography (TVS) unless otherwise stated.

Role of Imaging

Prenatal Screening

Alpha-Fetoprotein (AFP) ( Fig. 10.2 )

• • AFP is formed by the fetal liver, yolk sac, and gut and is found at different concentrations in fetal serum, amniotic fluid, and maternal serum (MSAFP).

  • In the normal fetus, AFP originates from fetal serum and enters amniotic fluid through fetal urination, fetal gastrointestinal (GI) secretions, and transudation from membranes (amnion and placenta).

  • Elevated MSAFP levels occur if there is transudation of AFP into the maternal serum, such as in open neural tube defects (NTDs) (AFP screening has an 80%–90% sensitivity for detection) or in fetal swallowing problems (abdominal wall defect).

  • MSAFP is best measured at 16 weeks.

  • False-positive causes of elevated AFP include:

    • GA 2 weeks ≥ to that estimated clinically

    • Multiple gestations

    • Fetal death

  • In cases of elevated AFP, test for acetylcholinesterase in amniotic fluid, which is present in NTDs.

Amniocentesis

Performed at 15–16 weeks using a ultrasound (US)-guided transabdominal approach. Desquamated cells of amniotic fluid are cultured and then karyotyped. In twin pregnancies, indigo carmine is injected into the amniotic cavity, punctured first to ensure sampling of both cavities. The main complication is fetal loss (1%).

Chorionic Villus Sampling (CVS)

• • Performed earlier than amniocentesis: 11–12 weeks

  • Transcervical or transabdominal approach under US guidance

  • Risk of fetal loss is 1%

NT Measurement

• • Can be measured between 11+0 and 13+6 GA

  • Measured from inner margin to inner margin

  • Neck must be in neutral position

  • Image must be midline (hard palate, nasal bone and hypoechoic diencephalon in view, no zygoma in view)

  • Fetal head and upper thorax must occupy entire field of view

  • If nuchal cord is present, mean of measurements above and below are taken

  • Measurement is used to assign a likelihood ratio (LR) for chromosomal abnormalities (in conjunction with the other aspects of the first-trimester screen). The higher the measurement, the higher the LR and vice versa.

  • Higher values are associated with:

    • Chromosomal abnormalities (21, 18, 13), 20%

    • Cardiac anomalies

    • Skeletal dysplasia

Approach to First-Trimester Sonogram ( Fig. 10.3 )

• CRL = Crown-rump length

• MSD = Mean sac diameter

Early Development ( Fig. 10.4 )

• • Fertilization occurs in fallopian tubes.

  • Oocyte + sperm cell = zygote.

  • Cleavage occurs in the fallopian tube.

  • Morula enters the uterine cavity.

  • Blastocyst implants into the endometrial wall.

  • Corpus luteum develops from ruptured graafian follicle (usually <2 cm), secretes progesterone, and induces decidual reaction.

  • Corpus luteum regresses at 10 weeks, when its function is taken over by the placenta.

Amniotic and Chorionic Membranes ( Fig. 10.5 )

• • Amniotic cavity is initially small.

  • Embryo lies within the amniotic cavity.

  • Amniotic cavity enlarges and ultimately fuses with the chorion (14–16 weeks).

  • Yolk sac is located outside the amniotic cavity but within the chorionic cavity; it is connected to primitive gut by the omphalomesenteric duct.

Double Decidual Sac Sign ( Fig. 10.6 )

• The double decidual sac sign is a useful early sign of IUP. It is based on the demonstration of three layers of different echogenicity:

  • Decidua parietalis (hyperechoic)

  • Fluid in the uterine cavity (hypoechoic)

  • Decidua capsularis (hyperechoic)

• The double-bleb sign refers to the presence of an amnion and yolk sac at 5–6 weeks. The embryo lies between these two structures.

Yolk Sac

• • Provides nutrients to the embryo until placental circulation is established.

  • Angiogenesis and hematopoiesis occur in the wall of the yolk sac.

  • Dorsal part is later incorporated into the primitive gut and remains connected by the omphalomesenteric (vitelline) duct.

Fetal Heart

A fetal heartbeat should always be detectable if the CRL is ≥7 mm by TVS. Absence of cardiac activity in embryos >7 mm is indicative of fetal demise.

Normal Gestational Sac

The gestational sac is the implantation product that occurs in the uterus on approximately day 21. At that time the blastocyst is approximately 0.1 mm in size and cannot be seen by US. Normal sacs become visible when they reach 2–3 mm. Measurements:

• Normal MSD (mm) + 30 = Days of pregnancy

After the gestational sac has developed, a yolk sac, the fetal heartbeat, and the embryo will become visible.

Order of appearance of structures: gestational sac → yolk sac → embryo (fetal pole) → amnion ( Fig. 10.7 )

Correlation of MSD and β-HCG Levels ( Fig. 10.8 )

• • β-HCG and MSD increase proportionally until the 8th week (25-mm MSD).

  • β-HCG doubles every 2–3 days.

  • β-HCG levels decline after 8 weeks.

  • Normal MSD growth: 1.1 mm/day

  • Discordance between MSD and β-HCG indicates an increased probability of demise.

Diagnosis of Pregnancy Failure

• US findings diagnostic of pregnancy failure:

  • CRL ≥7 mm and no cardiac activity

  • MSD ≥25 mm and no embryo

  • Absence of cardiac activity 14 days following previous US showing gestational sac without yolk sac.

  • Absence of cardiac activity 11 days following previous US showing gestational sac with yolk sac.

• US findings suspicious for, though not diagnostic of, pregnancy failure:

  • CRL <7 mm and no cardiac activity

  • MSD 16–24 mm and no embryo

  • Absence of cardiac activity 7–13 days following previous US showing gestational sac without yolk sac.

  • Absence of cardiac activity 7–10 days following previous US showing gestational sac with yolk sac.

  • Morphologic abnormalities

• Morphologic abnormalities suspicious for pregnancy failure:

  • Small gestational sac (MSD – CRL >5 mm)

  • Irregular shape of gestational sac or embryo

  • Large yolk sac (>7 mm)

  • Empty amnion

Terminology of Abortion

• Threatened abortion

  • Vaginal bleeding with closed cervical os during the first 20 weeks of pregnancy

  • Occurs in 25% of first trimester pregnancies

  • 50% survival

• Inevitable abortion

  • Vaginal bleeding with open cervical os; an abortion in progress

  • Incomplete abortion

  • Retained products of conception causing continued bleeding

  • Spontaneous abortion

  • Vaginal bleeding, passage of tissue

  • Most common in first trimester

  • No US evidence of viable IUP; ectopic pregnancy must be excluded.

  • High percentage have chromosomal abnormalities.

  • Missed abortion

  • Retention of a dead pregnancy for at least 2 months

The presence of cardiac activity indicates a good but not a 100% chance that a pregnancy will progress to term. There is still a 20% chance of pregnancy loss during the first 8 weeks even if a positive heartbeat is present. During the 9th–12th weeks, the chance of fetal loss decreases to 1%–2% in the presence of a positive heartbeat.

Subchorionic Hemorrhage

Venous bleeding causing marginal abruption with separation of the chorion from the endometrial lining extending to the margin of the placenta. Usually (80%) occurs in the late first trimester and presents as vaginal bleeding. Prognosis: generally good if there is a fetal heartbeat and bleeding is minimal. Hemorrhage greater than two-thirds of the chorionic sac circumference is associated with more than a two-fold increase in risk of pregnancy loss.

US Features ( Fig. 10.9 )

• Associated with marginal separation of placenta

• Hypoechoic or hyperechoic blood separates chorion from endometrium

• Distinguish from retroplacental bleed and abruption by location and extent of placental involvement

Location ( Fig. 10.10 )

• • Tubal, 97%

    • Ampullary (most common)

    • Isthmus

• Interstitial (cornual), 3%

• Ovarian, 1%

• Cervical (very rare)

• Fimbria (very rare)

Clinical Findings

• Incidence: 0.5%–1% of all pregnancies. Triad:

  • Pain, 95%

  • Hemorrhage, 85%

  • Palpable adnexal mass, 40%

Risk Factors

• • Previous ectopic pregnancy

  • History of pelvic inflammatory disease (PID)

  • Tubal surgery or other tubal abnormalities

  • Endometriosis

  • Previous pelvic surgery

  • Infertility and infertility treatments

  • Uterotubal anomalies

  • History of in utero exposure to diethylstilbestrol (DES)

  • Cigarette smoking

  • Intrauterine device (IUD) is not a risk factor but has been associated with ectopic pregnancy because IUD prevents IUP but not ectopic pregnancy.

Diagnostic Tests

• Serum markers

  • Normal β-HCG doubling time depends on the age of the pregnancy but on average is 2 days.

  • Ectopic pregnancies usually have a slower increase in β-HCG than normal pregnancies.

  • Low levels of β-HCG suggest ectopic pregnancy.

  • Reduced levels of progesterone P4 suggest ectopic pregnancy.

• Culdocentesis

  • Considered to be positive if >5 mL of nonclotted blood is aspirated; clotted blood indicates that a vessel has been entered; dry tap is nondiagnostic.

  • Culdocentesis is preferred to detect ectopic pregnancy of <6 weeks; at later time points, US is preferred.

• US

  • Always use TVS. If negative, also perform transabdominal ultrasound (TAS).

  • Doppler US can be used to detect peritrophoblastic flow.

US Features

• Uterus

  • May be normal

  • Thick decidual cast with no gestational sac ( Fig. 10.11 )

    

  • Pseudogestational sac ( Fig. 10.12 )

    

  • Fills endometrial cavity symmetrically

  • May be large

  • No double decidual sign but rather a single rim of echoes around pseudogestational sac

• Extrauterine structures ( Fig. 10.13 )

  • Free fluid in cul-de-sac (bleeding); may be anechoic or echogenic

  • Simple adnexal cyst (10% chance of pregnancy)

  • Complex adnexal mass (95% chance of ectopic)

  • Tubal ring sign (95% chance of ectopic): echogenic rim surrounding an unruptured ectopic pregnancy

  • Live embryo outside uterus; 100% specific but only seen in 25%

  

Types

• Dizygotic twins (fraternal), 70%

  • Independent fertilization of two ova

  • Always dichorionic, diamniotic; each ovum has its own placenta and amnion. Overall, 80% of twins are dichorionic, diamniotic.

  • Risk factors:

    • Advanced maternal age

    • Family history of twins

    • Ethnicity (e.g., Nigerian)

• Monozygotic twins (identical), 30%

  • Duplication of single fertilized ovum

  • May be monochorionic or dichorionic

  • Independent of maternal age, heredity, and race

Placental Unit ( Fig. 10.14 )

• • Amnionicity: number of amniotic sacs

  • Chorionicity: number of placentas

  • Dizygotic twins are always diamniotic, dichorionic (i.e., have two sacs and two placentas). The two placentas may fuse but do not have vascular connections.

  • Monozygotic twins have different amnionicity and chorionicity depending on the stage of cleavage of the single fertilized ovum.

  • The amnionicity/chorionicity determines the risk of complications:

    • Monoamniotic: > monochorionic, diamniotic > dichorionic

    • Monoamniotic: cord entanglement

    • Monochorionic: twin-twin transfusion syn­drome, twin anemia–polycythemia sequence

US Imaging

US Features

• Findings definitely indicating dichorionicity:

  • Separate placentas

  • Different fetal sex

  • Thick (≥2 mm) membrane separating twins in first trimester

  • Lambda sign: chorion extending into intertwin membrane

• Findings indicative of diamnionicity:

  • Thin membrane in first trimester

  • Two yolk sacs

In the second trimester, the sensitivity for finding an amnion is only 30%. In 70% of cases, an amnion is present but not visible.

Overview of Complications in Twin Pregnancies ( Fig. 10.15 )

• All twins

  • Increased incidence of premature labor

  • Fetal mortality three times higher than for single pregnancy

  • Neonatal mortality seven times higher than that for single pregnancy

  • Dichorionic, diamniotic twins

  • Perinatal mortality, 10%

• Monochorionic, diamniotic (MD) twins

  • Perinatal mortality, 20%

  • Twin-twin transfusion

  • Twin anemia–polycythemia sequence

  • Acardia

  • Demise of cotwin

  • Twin embolization syndrome

  • Structural abnormalities

• Monochorionic, monoamniotic (MM) twins

  • Perinatal mortality, 50%

  • Entangled cords

  • Conjoined twins

  • All the MD complications as well

Twin-Twin Transfusion Syndrome ( Fig. 10.16 )

Only occurs in monochorionic twins (25%). Results from arteriovenous communications in placenta. Very poor prognosis.

Conditions Associated With Demise of a Twin

Anatomy

Normal Central Nervous System (CNS) Structures ( Fig. 10.17 )

• • Ventricles: <10 mm at atrium

    • Choroid plexus in atria of lateral ventricles should occupy 60%–90% of atrium. It is easier to see the far field atrium as reverberation artifacts that may obscure near field atrium.

    • Choroid absent in anterior or occipital horns

  • Cisterna magna: 4–10 mm

  • Thalami are in midline

  • Cavum septum pellucidum

Signal Intensities ( Fig. 10.18 )

• Hyperechoic structures

  • Choroid plexus

  • Pia-arachnoid

  • Dura

  • Cerebellar vermis

  • Specular reflections from ventricles

• Hypoechoic structures

  • Brain white matter (WM)

  • Cerebrospinal fluid (CSF)

Spine ( Fig. 10.19 )

• • Three hyperechoic ossification centers

    • Posterior ossification centers: two neural arches

    • Anterior ossification center: vertebral body

  • Spinal cord is hypoechoic

Holoprosencephaly

• Failure of midline cleavage of the forebrain:

  • Alobar form: no cleavage

  • Semilobar form: partial cleavage

  • Lobar form: almost complete cleavage

  • Syntelencephaly or middle interhemispheric variant

US Features ( Fig. 10.20 )

• Alobar holoprosencephaly

  • Monoventricle communicates with dorsal cyst

  • Thin anterior mantle of brain tissue: “horseshoe” or “boomerang”

  • Fused thalami

  • No falx, corpus callosum, or septum pellucidum

  • No brain tissue around dorsal cyst

• Semilobar holoprosencephaly

  • Monoventricle with rudimentary occipital horns

  • Posterior brain tissue is present (no dorsal cyst).

  • Fused thalami

  • Partial falx posteriorly

  • No corpus callosum or septum pellucidum

• Lobar holoprosencephaly

  • Similar to semilobar, though more formed falx and greater midline hemispheric division

  • Very difficult to detect prenatally

• Syntelencephaly or middle interhemispheric variant

  • Mild form with abnormal midline connection of cortex anterior to formed falx

  • Not detectable prenatally

• All severe types have:

  • Absent septum pellucidum and corpus callosum

  • Thalamic fusion

  • Associated midline facial anomalies: clefts, cyclopia, hypotelorism

Agenesis of Corpus Callosum (ACC)

The normal development of the corpus callosum begins anterior (genu) and progresses to posterior (splenium). Agenesis may be partial (affects dysgenesis posterior aspects) or complete.

Hydranencephaly

• • Near-total absence of cerebrum with intact cranial vault, thalamus, and brainstem

  • Secondary to occlusion of internal carotid arteries (ICAs)

Porencephaly

• • Less severe vascular insult than in hydranencephaly

  • Cystic lesions are often in free communication with the ventricle and/or subarachnoid cisterns.

Ventriculomegaly ( Fig. 10.21 )

Refers to dilated (>10 mm) ventricles (measured at level of atrium). Associated with other anomalies in 75% of cases. Types:

• • Hydrocephalus (noncommunicating, obstructive > communicating)

  • Brain atrophy (enough brain tissue developed but it regresses later)

  • Colpocephaly (not enough brain tissue developed)

Hydrocephalus is the most common CNS abnormality. Causes include:

• Obstructive (common)

  • Spina bifida is the most common cause of hydrocephalus.

  • Aqueductal stenosis

  • DW syndrome

  • Encephalocele

  • Arnold-Chiari malformation

• Nonobstructive (uncommon)

  • Hemorrhage

  • Infection: cytomegalovirus (CMV), Toxoplasma (calcification)

  • Chromosomal abnormality (T21)

Cystic Structures

Hemorrhage

Similar imaging features and classification (Papile grades 1–4) to germinal matrix hemorrhage in fetuses born prematurely but different causes. In utero hemorrhage is very common. Causes:

• • Maternal hypertension (HTN), eclampsia

  • Isoimmune thrombocytopenia

  • Maternal hemorrhage

  • Nonimmune hydrops

DW Syndrome ( Fig. 10.22 )

Abnormal development of posterior fossa structures characterized by:

• • Posterior fossa cyst that communicates with the fourth ventricle

  • Hypoplasia of the cerebellar vermis

  • Variable hydrocephalus

  • Variant form does not have enlargement of posterior fossa.

Large Cisterna Magna ( Fig. 10.23 )

Diagnosis of exclusion; must exclude DW complex.

Neural Tube Defect (NTD)

Incidence: 1 : 600 births in the United States. Increased risk (3%) in parents with previous NTD child. Screening: amniotic fluid and MSAFP are increased because of transudation of fetal serum AFP across the NTD. Spectrum of disease:

• • Anencephaly (most common)

  • Spina bifida and meningomyelocele

  • Face and orbits usually intact

  • Encephalocele (least common)

Anencephaly ( Fig. 10.24 )

• • Complete absence of cranial vault (acrania) and cerebral hemispheres; should be symmetric. Asymmetric absence should raise the suspicion of amniotic band syndrome (ABS).

  • Angiomatous tissue covers base of the skull

  • Some functioning neural tissue is nearly always present.

  • Polyhydramnios, 50%

  • Should not be diagnosed before 14 weeks of age (skull is not ossified)

Encephalocele ( Fig. 10.25 )

Herniation of intracranial structures through a cranial defect. Cephalocele = meninges; encephalocele = brain and meninges. Most defects are covered by skin, and MSAFP levels thus are normal. Location: occipital, 70%; frontal, 10%. Lesions are typically midline. Asymmetric lesions should raise the suspicion of ABS. Prognosis depends on the amount of herniated brain. Mortality, 50%; intellectual impairment, 50%–90%.

Spina Bifida and Myelomeningocele ( Fig. 10.26 )

Location: lumbosacral > thoracic, cervical spine. MSAFP is elevated unless the myelomeningocele is covered with skin. Incidence: 0.1% of pregnancies.

• Associations (as a result of imbalanced muscular activity):

  • Clubfoot

  • Hip dislocations

Algorithm for Intracranial Malformations ( Fig. 10.29 )

Cystic Hygroma

Fluid-filled structures with spokewheel appearance caused by lymphatic malformation. Location: neck, upper thorax. Prognosis depends on size: high incidence of hydrops and in utero death in large lesions. Cystic hygromas in noncervical location do not carry a significant risk of chromosomal anomalies and have a favorable outcome. Amniocentesis is performed in cervical cystic hygromas because of frequently associated syndromes:

• • Turner, 45XO (most common, 50%)

  • Trisomies (21, 18, 13)

  • Noonan syndrome

  • Fetal alcohol syndrome

Other Anomalies of the Face and Neck

Detection

Cardiac abnormalities are often difficult to detect because of small heart size, complex anatomy, and rapid heart rate (HR). Because cardiac abnormalities may be associated with chromosomal abnormalities (15%–40%), amniocentesis is indicated in all patients with cardiac defects.

• Cardiac abnormalities best detected on four-chamber view:

  • Septal defect–ventricular septal defect (VSD), arteriovenous canal

  • Endocardial cushion defect

  • Hypoplastic left heart: absent or small LV

  • Ebstein anomaly (associated with maternal lithium use): large right atrium (RA) and small RV; tricuspid valve within RV

  • Critical aortic stenosis: RV < LV. Coarctation: LV < RV

• Cardiac abnormalities best detected on outflow tract views:

  • Tetralogy of Fallot: large aorta overriding a small PA

  • Transposition of great arteries (TGA): large vessels run in a parallel plane

  • Truncus arteriosus: single truncal vessel overriding the septum

  • Pentalogy of Cantrell

    • Omphalocele

    • Sternal cleft

    • Cardiac exstrophy (ectopia cordis)

    • CVS malformations

    • Anterior diaphragmatic hernia

• Cardiac abnormalities often missed:

  • Isolated atrial septal defect (ASD)

  • Isolated VSD

  • Aortic or pulmonic stenosis

  • Coarctation of the aorta

  • Total anomalous pulmonary venous return (TAPVR)

• Other detectable abnormalities:

  • Rhabdomyoma: most common prenatal and neonatal cardiac tumor (commonly associated with tuberous sclerosis).

  • Endocardial fibroelastosis: markedly echogenic myocardium

  • Ectopia cordis: heart is outside thoracic cavity

  • Cardiomyopathy: dilated heart, poor contractility

Maternal Risk Factors for CHD

• • Diabetes

  • Infection: rubella, CMV

  • Collagen vascular disease: systemic lupus erythematosus (SLE)

  • Drugs: alcohol, trimethadione, phenytoin, lithium

  • Family history of heart disease

Fetal Arrhythmias (Use M-Mode or Doppler US for Evaluation)

• • Premature atrial contractions (PACs) are the most common fetal arrhythmia.

  • PACs and premature ventricular contractions (PVCs) are benign (most disappear in utero).

  • Supraventricular tachycardia (HR ≥180 BPM) is the most common tachyarrhythmia:

    • 10% incidence in CHD: structural abnormalities uncommon

    • May lead to hydrops. Treatment is with digoxin or verapamil.

  • Fetal bradycardia (HR <100 BPM for >10 seconds) usually indicates fetal hypoxia distress.

  • Fetal heart block: 40%–50% have structural abnormality.

    • 40% incidence in CHD

    • Associated with maternal SLE

Pulmonary Hypoplasia

Congenital Pulmonary Airway Malformation (CPAM)

A type of bronchopulmonary foregut malformation. Usually unilateral, involving one lobe.

Bronchopulmonary Sequestration

Only the extralobar type is usually detected prenatally.

Congenital Diaphragmatic Hernia (Bochdalek Hernia)

90% are on the left side; 95% are unilateral. Mortality: 50%–70% (because of pulmonary hypoplasia). Because of commonly associated anomalies, all patients with CDH should have an amniocentesis.

US Features ( Fig. 10.30 )

• Chest

  • Stomach and/or bowel adjacent to heart (key finding) on four-chamber view

  • Herniation into chest may occur intermittently.

  • Peristaltic movements in chest

  • Shift of heart and mediastinum

• Abdomen

  • Absent stomach in abdomen

  • Small abdominal circumference (because of herniation of organs into chest)

• Other

  • Polyhydramnios (impaired swallowing)

  • Always look for associated anomalies (anencephaly is most common).

Mediastinal Masses

• Anterior and middle mediastinum

  • Teratoma

  • Cystic hygroma

  • Normal thymus

• Posterior mediastinum

  • Neurogenic tumors

  • Enteric cysts

Pleural Effusion