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The goals of prenatal care are to assess pregnancy and fetal
risks and to monitor the pregnancy to optimize the chances for a
good outcome. Ideally, a pregnancy is planned, and counseling and
health assessments can begin before the fetus is conceived. There
are risk assessment tools for evaluating the mother’s family
and medical history, previous pregnancy history, and the progress
of this pregnancy.1 Currently, most women receive
a level 1 fetal ultrasound before 20 weeks’ gestation to
assess gestational age, screen the fetus for anomalies, determine
placental position, and identify multiple births. A level 1 ultrasound
obtained prior to 20 weeks is more accurate than the date of the
last menstrual period to determine gestational age. The early screening
ultrasound is not intended to identify more subtle structural abnormalities. If
anomalies are noted, the woman will be referred for a complete ultrasound
evaluation of the fetus using more sensitive equipment and expert
evaluations. In the United States it is now unusual for infants
to deliver with major skeletal, cardiac, or other internal organ
structural anomalies that have not been identified prior to delivery.
Common chromosome anomalies such as trisomy 21 also have structural
signatures that can be identified by early gestational ultrasound.
The genetic abnormality then can be verified by amniocentesis. An accurate
early assessment of gestational age is the cornerstone for subsequent
monitoring of the pregnancy and decisions about the timing of delivery.
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All women should have plasma tests to evaluate blood type and
Coombs status. Women who are Rh negative are carefully followed and
given anti-Rh antibodies for fetal loss and for pregnancy abnormalities
that may be associated with bleeding. The maternal plasma also is
tested for antibodies to syphilis, rubella, hepatitis B, and often
human immunodeficiency virus (HIV), as these infectious diseases
adversely affect the fetus. Women are routinely offered second-trimester
antenatal screening for Down syndrome using 3 analytes in maternal
plasma, called the triple screen.2 Women at high
risk of having a fetus with Down syndrome will have low α-fetoprotein,
high human chorionic gonadotrophin, and low unconjugated estriol
levels, adjusted for gestational age. If the maternal plasma tests
indicate increased risk, a level 2 ultrasound can identify structural
findings frequently associated with Down syndrome: increased nuchal
fold translucency, a hypoplastic fetal nasal bone, and a short length
relative to head size, together with other findings. The diagnosis
can be confirmed by karyotype of fetal cells from amniotic fluid. The
risk of Down syndrome increases with maternal age, and a fetal chromosome
assessment is recommended for pregnant women 35 years or older.
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Maternal serum α-fetoprotein values also are
used to screen pregnancies for neural tube defects. In contrast
to Down syndrome, this protein is elevated with open neural tube
defects as well as with abdominal wall defects such as omphalocele
or gastroschisis. Specific genetic screening is offered to women
on the basis of family history and racial background. These tests
can be performed on the ...