Transplacental infections gain access to the fetal bloodstream by crossing the placenta via the chorionic villi, and may occur at any time during gestation or occasionally, as may be the case with hepatitis B and human immunodeficiency virus, at the time of delivery via maternal-to-fetal transfusion. Most parasitic (e.g., toxoplasma, malaria) and viral infections, and a few bacterial infections (i.e., Listeria and Treponema), follow this mode of hematogenous transmission. The clinical manifestations of these infections are highly variable, depending largely on the gestational timing and the microorganism involved. The most important trans-placental infections can be conveniently remembered by the acronym TORCH. The elements of the TORCH complex are Toxoplasma (T), rubella virus (R), cytomegalovirus (C), herpesvirus (H), and any of a number of other (O) microbes such as Treponema pallidum. These agents are grouped together because they may evoke similar clinical and pathologic manifestations. TORCH infections occurring early in gestation may cause chronic sequelae in the child, including growth restriction, mental retardation, cataracts, and congenital cardiac anomalies, whereas infections later in pregnancy result primarily in tissue injury accompanied by inflammation (encephalitis, chorioretinitis, hepatosplenomegaly, pneumonia, and myocarditis). More recently, Zika virus has emerged as another agent that can be transmitted by pregnant females to their offspring with devastating consequences such as microcephaly and brain damage.
Prematurity is defined by a gestational age less than 37 weeks and is the second most common cause of neonatal mortality (second only to congenital anomalies). As might be expected, infants born before completion of gestation also weigh less than normal (<2500 gm). The major risk factors for prematurity include preterm premature rupture of membranes; intrauterine infection leading to inflammation of the placental membranes (chorioamnionitis); structural abnormalities of the uterus, cervix, and placenta; and multiple gestation (e.g., twin pregnancy). Children born before completion of the full period of gestation have higher morbidity and mortality rates than full-term infants. The immaturity of organ systems in preterm infants makes them especially vulnerable to several important complications:
Respiratory distress syndrome, also called hyaline membrane disease
Long-term sequelae, including developmental delay
Although birth weight is low in preterm infants, it usually is appropriate after adjustment for gestational age. By contrast, as many as one-third of infants who weigh less than 2500 gm are born at term and are therefore under-grown rather than immature. These small-for-gestationalage (SGA) infants suffer from fetal growth restriction. Fetal growth restriction may result from fetal, maternal, or placental abnormalities, although in many cases the specific cause is unknown.
Fetal abnormalities: This category consists of conditions that intrinsically reduce growth potential of the fetus despite an adequate supply of nutrients from the mother. Prominent among such fetal conditions are chromosomal disorders, congenital anomalies, and congenital infections. Chromosomal abnormalities may be detected in as many as 17% of fetuses evaluated for fetal growth restriction and in as many as 66% of fetuses with documented ultrasonographic malformations. Fetal infection should be considered in all growth-restricted neonates, with the TORCH group of infections (see earlier) being a common cause. When the causation is intrinsic to the fetus, fetal growth restriction is symmetric (i.e., affects all organ systems equally).