Parental care is an ongoing process of health optimization for the woman and her fetus and requires continual assessment of medical and social determinants of health. Parental care is associated with improved reproductive outcomes including decreases in preterm birth, fetal growth restriction, and neonatal death. See Table 1-3 for recommendations for routine prenatal tests.
It is important to establish the correct gestational dating of a pregnancy as soon as possible during prenatal care. This can influence interpretation of antenatal testing and determine optimal delivery timing. Assuming that ovulation and conception occurred on the 14th day of a 28-day cycle, the average length of a human pregnancy is 280 days, counting from the first day of the last menstrual period (LMP). As soon as data from the LMP and first ultrasound are obtained, an estimated date of delivery (EDD) should be established and clearly communicated with the patient.
Pregnancies resulting from assisted reproductive technology should use the assisted reproductive technology–guided gestational age for establishing the EDD.
Naegele's rule. To estimate the date of delivery, determine the first day of the LMP, add 7 days, then add 1 year, and then subtract 3 months.
Ultrasonographic dating. If EDD by ultrasound measurements falls within the range of accuracy, the LMP is used to establish the EDD as confirmed by ultrasound (Table 1-4).
Nutrition. A pregnant woman requires approximately 15% more calories than she does when not pregnant, that is, typically an additional 300 to 500 kcal/d.
Iron. Consumption of iron-rich foods is encouraged throughout pregnancy. The National Academy of Sciences recommends adding 27-mg iron supplementation (typically present in prenatal vitamins) to the average diet. Additional supplementation may be required.
Weight gain. Recommendations regarding total weight gain in pregnancy aim to optimize maternal and fetal outcomes. These recommendations are based on prepregnancy body mass index (BMI) and were established by the Institute of Medicine (Table 1-5).
Obesity (BMI > 30 kg/m2) is associated with an increased risk for poor pregnancy outcomes including miscarriage, stillbirth, fetal anomalies, preterm delivery, gestational diabetes and hypertension, preeclampsia, thromboembolic event, cesarean delivery, and shoulder dystocia. Women with a history of gastric bypass surgery should be evaluated for nutritional deficiencies (eg, protein, iron, vitamin B12, folate, vitamin D, calcium). Weight loss counseling and referral to a nutritionist should be offered.
Physical exercise. The US Department of Health and Human Services recommends that women participate in at least 150 minutes of moderate physical activity per week during pregnancy and the postpartum period. This should be continued as tolerated during the pregnancy. Absolute contraindications to aerobic exercise during pregnancy include severe cardiac or pulmonary disease, cervical insufficiency or cerclage, multiple gestation at risk of premature labor, persistent second- or third-trimester bleeding, placenta previa, ruptured membranes, pregnancy-induced hypertension, and severe anemia. Relative contraindications to aerobic exercise during pregnancy include poor health status prior to pregnancy, anemia, unevaluated maternal cardiac arrhythmia, lung disease, poorly controlled type 1 diabetes, extremes of maternal weight, orthopedic or neurologic limitations, poorly controlled thyroid disease, and fetal growth abnormalities. Contact sports and activities with increased risk of falling or involving extremes in temperature should be avoided.
All pregnant women should be offered testing to assess risk of fetal aneuploidy. Fetal aneuploidy refers to conditions associated with an abnormal number of chromosomes. Whereas chromosomal abnormalities occur in approximately 1 in 150 live births, the prevalence is greater as aneuploidy accounts for a large proportion of early pregnancy loss.
Risk factors to consider for referral to genetic counseling (Table 1-6) include family history, maternal age, ethnicity, drug and environmental exposures, and obstetric and medical history.
Providing access to prenatal genetic counseling and screening allows providers to provide adequate prenatal interventions if needed, optimize neonatal outcomes by organizing appropriate resources following delivery, and provide relative reassurance to parents when testing is normal.
ACOG recommends providing universal carrier screening for cystic fibrosis and spinal muscular atrophy to women who are considering pregnancy or are currently pregnant. Case-specific carrier screening can be offered for Tay-Sachs disease, fragile X syndrome, and certain hemoglobinopathies.
Down syndrome (trisomy 21) is the most common condition associated with an abnormal chromosome number found in live born children that is not related to a sex chromosome disorder. All cases are identified by the presence of an extra chromosome 21, which can result from nondisjunction, translocations, or mosaicism. Down syndrome can vary in presentation and is associated with hypotonia, characteristic facial features, congenital heart defects, intellectual disability, and intestinal atresia (see chapter 9).
Patau syndrome (trisomy 13) and Edward syndrome (trisomy 18) are more severe disorders associated with multiorgan birth defects and intellectual disability. The majority of cases die in utero or within the first year of life.
See Table 1-7 for a summary of the common prenatal genetic screening tests.
First trimester screening is performed between 10 0/7 weeks' and 13 6/7 weeks' gestation. The test includes nuchal translucency, maternal age, and two maternal serum markers: free β-human chorionic gonadotrophin (hCG) and pregnancy-associated plasma protein-A.
Factors required for accurate risk calculation include a history of prior birth with aneuploidy, race, number of fetuses, and current maternal weight.
The detection rate for trisomy 21 is approximately 82% to 87% with a 5% false-positive rate, which is improved by adding an ultrasound assessment of the fetal nasal bone (about 95%). Detection rate for trisomy 18 is about 95%.
First trimester screening does not include assessment of risk for NTDs.
The quad screen is typically performed between 15 0/7 weeks' and 22 6/7 weeks' gestation and can establish risk for trisomy 18 and 21 and NTDs.
Four maternal serum markers are evaluated: hCG, maternal serum α-fetoprotein (MSAFP), unconjugated estriol (uE3), and dimeric inhibin A (DIA). Maternal age is also noted during the evaluation.
Trisomy 21 is detected at a rate of 75% for women younger than 35 years and 90% for those older than 35 years.
Elevated α-fetoprotein is associated with defects of the abdominal wall or NTDs.
Combined screening uses combined first and second trimester screening to adjust a woman's age-related risk for fetal aneuploidy.
Integrated screening uses nuchal translucency and pregnancy-associated plasma protein-A from the first trimester screening as well as hCG, MSAFP, uE3, and DIA from the second trimester screening. Results are reported only after both screening tests are completed. The detection rate for this method is 94% to 96% with 5% false positives; this is equivalent to first trimester screening when nasal bone is included in the risk assessment.
Sequential screening involves giving the patient the first trimester screen results. If at high risk, patients are given the option for invasive testing, whereas those at low risk can still undergo second trimester screening to achieve a higher detection rate.
Fragments of fetal DNA can be isolated from maternal blood to determine the risk for several fetal conditions. This test can be performed between 10 weeks' gestation and term and detects >98% of trisomy 21 pregnancies. The false-positive rate is <0.5%, although this rate is higher in women at low risk of Down syndrome. The rate of detection is lower for trisomy 13 and 18. Women with no reportable result are also at increased risk of fetal aneuploidy.
NTDs are the second most common type of congenital abnormality, after cardiac anomalies. This group of disorders is characterized by failure of the neural tube to close or be sealed by normal musculoskeletal coverings during embryogenesis. Incomplete neural tube closure can result in a range of disorders including meningocele, myelomeningocele, or anencephaly. Depending on the severity, some defects may have potential for surgical correction. The NTDs can be isolated defects or associated with syndromes and are associated with certain environmental disorders, maternal diseases, and ethnicities. Primary prevention of NTDs is recommended with folic acid supplementation (see chapter 9).
Screening for NTDs should be offered to all pregnant women either with second trimester MSAFP and/or targeted fetal anatomy ultrasound.
The MSAFP is a glycoprotein secreted by the fetal liver and yolk sac. Although not diagnostic, abnormally elevated levels of MSAFP (greater than 2.5 multiples of the median) are suspicious for open NTDs. Elevated MSAFP can also be detected in pregnancies complicated by abdominal wall defects, cystic hygroma, teratomas, fetal demise, multiple gestation, or incorrect pregnancy dating, whereas abnormally low levels of α-fetoprotein may be associated with fetal aneuploidy. Diagnostic ultrasonography should be performed in cases with abnormal MSAFP.
Ultrasound evaluation of fetal anatomy and placental location should be performed between 18 and 22 weeks' gestation. Midtrimester ultrasonography seeks to identify major structural abnormalities (eg, enlarged nuchal translucency, cystic hygroma, cardiac malformations) and ultrasonographic “soft markers” of aneuploidy (eg, thickened nuchal fold, pyelectasis, echogenic bowel, short femur length). However, the sensitivity of ultrasound is widely variable depending on ultrasonographer experience, machine quality, number of fetuses, and maternal BMI. Women with high a priori risk of fetal aneuploidy should be offered diagnostic testing even when fetal anatomy is thought to appear normal on ultrasound evaluation.
Parental diagnostic testing is most commonly done to evaluate for fetal chromosomal abnormalities. Fetal cells obtained by either chorionic villus sampling (CVS) or amniocentesis can then be evaluated using traditional karyotype, fluorescence in situ hybridization, or chromosomal microarray analysis. Unsensitized Rh-negative women undergoing prenatal diagnostic testing should receive Rho(D) immune globulin.
In chorionic villus sampling, placental tissue is retrieved via transcervical or transabdominal aspiration.
This procedure is most commonly performed between 10 and 13 weeks of gestation, although the transabdominal approach may be offered throughout the second and third trimesters. There is no significant difference in the risks of the two approaches.
The CVS can be performed at an earlier gestational age than amniocentesis, allowing earlier diagnosis and thus earlier option for pregnancy termination.
The risk of CVS-related pregnancy loss is approximately 1:455 (0.22%).
In 1991, reports of limb defects after CVS were first reported. The risk of this outcome is decreased when the procedure is performed after 10 weeks' gestation.
Other risks of CVS include vaginal spotting, which is more frequent with the transcervical approach. Amniotic fluid leak or infection after CVS is very rare.
Unsensitized Rh-negative women undergoing CVS should receive Rh immunoglobin.
Amniocentesis is the transabdominal aspiration of 20 to 30 mL of amniotic fluid. Amniotic fluid contains fetal cells shed from the fetal gastrointestinal tract, skin, and bladder.
This procedure is typically performed between 15 and 20 weeks' gestation but can also be performed at more advanced gestational ages.
Placental puncture should be avoided because fetal blood can contaminate the specimen and lead to false-positive results.
Although accurate data are difficult to obtain, the rate of procedure-related pregnancy loss attributable to prenatal diagnostic procedures is estimated to be 0.1% to 0.3% when performed by experienced health care providers, and overall, the pregnancy loss rate for amniocentesis is very low. Other complications, including vaginal bleeding, leakage of amniotic fluid, and fetal trauma, occur infrequently, approximately 1% to 2% of all cases.