Historically, pregnancy was a very tenuous part of a woman's life because of the high mortality rate associated with gestation. In the early 1900s, maternal mortality was 6 to 9 out of 1000 live births, and 100 per 1000 live-born infants died within the first 12 months of life. In 2000, maternal mortality declined to less than 0.1 out of 1000 live births, and infant mortality declined to 7 out of 1000 live births. Improvements in nutrition and advances in clinical medicine are key factors in the decline in mortality. Today, prevention of mortality is not the only goal but also the prevention of morbidity. Prenatal screening helps practitioners understand the physical needs of a gestating mother to prevent both morbidity and mortality. 
The first 14 weeks of pregnancy are exciting for parents but an important time for screening by their practitioners. The first obstetrical visit should include a general physical examination and pelvic examination. Baseline vital signs along with the physical examination aid a practitioner in assessing the overall physical wellbeing of the gestating mother. At this time, chronic medical and psychological problems can be addressed by the practitioner as well as a prior obstetrical/gynecologic history. The pelvic examination should include cervical cytology (i.e., Pap smear) if one has not been collected within the past 12 months and the female patient is greater than or equal to 21 years old. Testing for Neisseria gonorrhea and Chlamydia trachomatis should also be performed. The uterus, adnexa, and cervix should be palpated in a bimanual pelvic examination to determine if masses or other abnormalities are present.
One of the most important things to determine during the first trimester is a gestational age and estimated due date. Often, the gestational age is calculated from the last menstrual period if it is known by the patient. If the last menstrual period is not known or the uterine size does not correlate with the estimated due date by the last menstrual period, an ultrasound for dating takes place. Ultrasound in the first trimester gives the most accurate gestational age and estimated due date. The ultrasonographer will assess live fetal status by measuring cardiac activity as well as assess the number of fetuses, amniotic sacs, and placentas. The ultrasonographer will calculate an estimated gestational age by measuring the crown-rump length of the fetus. This examination will also detail if uterine or adnexal abnormalities are present.
Laboratory tests are often performed during the first prenatal visit. An assessment of Rh factor and abnormal antibodies, serological tests for syphilis and rubella and either a hemoglobin or hematocrit are required in most institutions. The American College of Obstetricians and Gynecologists (ACOG) also recommends screening for hepatitis B and HIV. Patients with chronic medical conditions should have additional testing specific for the evaluation of that condition. Aneuploidy screening can begin as early as ten weeks gestation with cell-free DNA in women with prior aneuploidy and women =35 years old. First-trimester aneuploidy screening generally occurs between weeks 11-13 and includes nuchal translucency measurement on ultrasound and maternal serum-free BhCG and PAPP-A to detect Down's syndrome and Trisomy 18.
Laboratory and ultrasound assessments of fetal genetic disorders and structural abnormalities typically take place between 15-20 weeks gestation in the second trimester. Since 1984, screening for Down's syndrome has been primarily maternal serum alpha protein levels (MSAFP) levels since a report then found that there was lower MSAFP found in women carrying a Down's syndrome affected fetus. Elevated hCG, elevated inhibin A and lower levels of unconjugated estriol are also linked with an increased risk of a Down's syndrome affected fetus creating the quad screen. Elevated MSAFP is associated with fetuses affected by open spina bifida. More recently, screening for genetic disorders trisomies 13, 18 and 21 have been performed using maternal serum cell-free fetal DNA which is considered a non-invasive prenatal test (NIPT). In 2012, ACOG published guidelines on NIPT use which includes women with prior aneuploidy and women =35 years old.
According the American Institute of Ultrasound in Medicine (AIUM), second trimester ultrasound assessments should include: (1) fetal cardiac activity, number, presentation; (2) estimate of amniotic fluid volume; (3) placental location, appearance, and relationship to the internal cervical os; (4) imaging of the umbilical cord, and the number of vessels in the cord; (5) gestational (menstrual) age (via biparietal diameter, head circumference, femoral diaphysis length, abdominal circumference, or average abdominal diameter); (6) fetal weight estimation; (7) maternal anatomy (uterus, adnexal structures, cervix); (8) and fetal anatomic survey. The fetal anatomic survey includes assessment of: head, face, neck, chest (including a four-chamber view of the fetal heart), abdomen, stomach, kidneys, bladder, abdominal cord insertion site, spine, extremities, and sex.
In the late second trimester (26-28 weeks gestation), most institutions will perform and oral glucose tolerance test (OGTT) to screen for gestational diabetes. The testing can be done with either a 1 hour 50-gram glucose load or a diagnostic 2 hour 75-gram glucose load.
Third-trimester prenatal testing generally focuses on maternal wellbeing and reducing fetal morbidity/mortality. Streptococcus agalactiae (group B streptococci beta-hemolytic, GBS) is a major cause of neonatal morbidity and mortality. Vaginal screening for GBS typically takes place in the late third trimester (34-37 weeks gestation) so that mothers positive for the bacterium can receive treatment during labor before delivery. Many institutions also require repeat evaluations of hemoglobin/hematocrit, syphilis serology, and HIV screening. 
Patients with a higher risk of genetic disorders based on ethnicity also should be screened in the first trimester. Cystic fibrosis (CF) is one of the most common genetic diseases in Caucasians and relatively common in Ashkenazi Jews and Hispanics. The National Institutes of Health (NIH) issued a consensus statement in 1997 indicating that CF testing should be offered to adults with a family history of CF, to partners of people with CF, to all couples planning a pregnancy, and to couples seeking prenatal testing. Patients of Ashkenazi Jew descent are predisposed to several genetic disorders. Both the American College of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics and Genomics (ACMG) recommend carrier testing for three carrier tests: Tay-Sachs disease, Canavan disease, and familial dysautonomia along with CF testing in this population. The ACMG also recommends screening for Fanconi anemia group C, Niemann-Pick type A, Bloom syndrome, mucolipidosis type IV, and Gaucher disease to all Ashkenazi Jewish couples. Screening for sickle cell disease should be offered to individuals of African, African-American, Mediterranean basin, Middle Eastern, and Indian descent.
At every prenatal visit in the first trimester, maternal vital signs, maternal weight, and urinalysis for glucose, protein, and leukocytes should be evaluated.
At every prenatal visit in the second trimester, maternal vital signs, maternal weight, fetal heart rate, uterine fundal height and urinalysis for glucose, protein, and leukocytes should be evaluated.
At every prenatal visit in the third trimester, maternal vital signs, maternal weight, fetal heart rate, uterine fundal height and urinalysis for glucose, protein, and leukocytes should be evaluated.
The key to prenatal screening is patient education provided by an interprofessional team. Besides the general healthcare provider, the obstetrician, dietitian and prenatal nurse should educate the patient on the benefits of prenatal screening. In order for patients to make informed decisions about the wide number of test, they need to understand the limitations and benefits of the testing. The patient should also be informed about the risk of some prenatal tests like amniocentesis or chorionic villus sampling. It is also important to educate the patient on the possibility of false positives and false negative results especially when testing for genetic or chromosomal disorders. Finally, genetic counselors should always be available when a diagnosis of a chromosomal disorder is diagnosed prenatally. The decision to continue with the pregnancy, adopt or choosing to end the pregnancy is not always easy and is best done with the help of genetic and mental health counselors. (Level V)
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