The Role of Ultrasound in the Management of a Twin Gestation with One Fetus Affected with Anencephaly

Abstract

Anencephaly is the most common type of neural tube defect seen during prenatal development. This condition results from a failure of the neural tube to close at its cephalad end between the second and third weeks of embryonic development. 1 Sonographically, anencephaly presents as the absence of the cranial vault and cerebral hemispheres. It is believed to be multifactorial in origin but usually not associated with chromosomal abnormalities. 2The condition is invariably lethal; 50% are stillborn, and the remainder die in the newborn period. 3We present a case of a twin gestation, with one fetus affected with anencephaly and the other with no apparent abnormalities.A 32-year-old gravida 2, para 1, woman was referred for an ultrasound examination at 21 weeks' gestation, by last menstrual period. A twin gestation was visualized with what appeared to be one placenta. A thin membrane was seen dividing the gestational sacs. Both fetuses were male. The presenting twin (twin A) was cephalad and very low in the material pelvis, with its spine to the maternal left. Twin B was cephalad with its spine to the maternal right (Fig. 1).Fetal heart motion was detected in both fetuses.Additionally, a slight increase in amniotic fluid was noted.Twin A's head was not well visualized secondary to its position deep in the maternal pelvis. Slight pressure was applied to the maternal abdomen in an attempt to elevate the head. This enabled a limited evaluation of the face and cervical spine area, which revealed very pronounced and protruding orbits. Saggital views of the cervical spine revealed a small portion of tissue at the base of the skull and the lack of a cranial vault (Fig. 2). Downloaded from 199 Fig. 1. A thin membrane was seen between the fetuses.In order to better visualize the defect and evaluate the cervix, a transvaginal ultrasound evaluation was performed. This revealed a complete absence of the cranial vault, no brain tissue, and a small remnant of the brain stem. The orbits were indeed protruding with a lack of ossified bone above them (Fig. 3). These findings were consistent with anencephaly.Transabdominally, the spine of the affected fetus was evaluated carefully for associated defects but appeared unremarkable. Saggital view of the face revealed the presence of what appeared to be lowset ears (Fig. 4), a phenomena occurring in 10-20% of affected fetus Adequate measurement of the fetal abdomen and femur were obtained. These were consistent with 20 weeks' gestational age.Twin B was also thoroughly evaluated. All anatomy of this fetus, including the intracranial contents, were within normal limits. Biometry of this fetus also measured 20 weeks' gestational age.The patient was subsequently sent for a confir-