obstetric and neonatal practice and technology (e.g.,
surfactant, steroids, ultrasound), may lead to earlier
diagnosis of problems and resultantly to an earlier ce-
sarean delivery, which, while further decreasing the
risk of maternal, fetal, and even infant mortality, may
potentially increase the rate of moderate preterm de-
livery. In all, the rise in preterm birthrates in the
United States during the 1990s may stem largely from
attempts to improve reporting, fertility, and survival
rather than from a major rise in high-risk behavioral
and medical factors.
The limited success of the efforts to reduce rates
of preterm birth in part stems from preterm birth
being a single outcome (i.e., being born too early) that
results from multiple causes, most of which are still
poorly understood. Only between 25 and 40 percent
of preterm births can be explained with currently
known risk factors, including single marital status;
low socioeconomic status; previous preterm birth;
maternal illness (e.g., hypertension); cocaine and to-
bacco use; multiple second trimester spontaneous
abortions; gestational bleeding; urogenital infections;
multiple gestations; placental, cervical, and uterine
abnormalities; and black race of mother (which may
reflect a complex array of socioeconomic, cultural, bi-
ological, and behavioral risk characteristics). The
major clinical classifications of preterm birth are
spontaneous preterm labor, preterm rupture of
membranes before the onset of labor, and indicated
preterm delivery for pregnancy complications. Nev-
ertheless, as each of these clinical presentations have
multiple causes, more recent efforts have focused on
establishing the many unique antecedents and biolog-
ical causes for preterm birth. Accordingly, the discov-
ery and development of a single prevention or
intervention strategy to markedly reduce the present
level of preterm births is highly unlikely and probably
unrealistic. In spite of the many risk factors for pre-
term birth that have been identified, only a few of
these risk factors, such as cigarette smoking, can be
considered modifiable during the current pregnancy.
In the early twenty-first century, research efforts to
further understand the determinants of preterm birth
are focusing on the role of infections, stress, socioeco-
nomic deprivation, pregnancy anxiety, hormones,
nutrition, and fetal growth restriction.
See also: BIRTHWEIGHT; HIGH RISK INFANTS;
INFANT MORTALITY; PREGNANCY
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Greg R. Alexander
Mary Ann Pass
Martha SlayPRENATAL CARE
Prenatal care refers to medical care and other health-
related services offered during pregnancy to ensure
the well-being of the mother and her future offspring.
Medical visits for prenatal care follow the pattern rec-
ommended by the American College of Obstetricians
and Gynecologists (ACOG): an initial visit in the first
trimester, one visit every four weeks through twenty-
eight weeks of gestational age, then a visit every two
weeks until thirty-six weeks, and then a visit every
week through forty weeks (or until delivery). This pat-
tern results in thirteen prenatal care visits for a nor-
mal length pregnancy. The emphasis on more visits324 PRENATAL CARE