Clinical laboratory testing is useful for the diagnosis and management of pregnancy and infertility, and such testing is reviewed in this chapter. Gestational diabetes mellitus (GDM) is discussed in Chapter 17, and hemolytic disease of the newborn (HDN) is found in Chapters 7 and 12. Female physiology and biochemistry including amenorrhea are discussed in Chapter 22. The female genital tract is also a common site for infections, which may be sexually transmitted, and it is a common site for tumors. The infections are presented in Chapter 5, and tumor descriptions are found in textbooks of anatomic pathology.

Description

Normal pregnancy lasts approximately 40 weeks, as dated from the first day of the previous menstrual period, and is typically divided into 3 intervals or trimesters each lasting approximately 13 weeks. Approximately 5 days after fertilization, a blastocyst implants in the uterus. Trophoblast cells of the blastocyst invade the endometrium with chorionic villi leading to a placenta and the forming embryo surrounded by amniotic fluid. The placenta nourishes the embryo and produces hormones vital to pregnancy such as human chorionic gonadotropin (hCG), progesterone, estradiol, estriol, and estrone. The amniotic fluid protects the embryo and changes composition as the pregnancy progresses. The embryo undergoes rapid cell division, differentiation, and growth in the first trimester (0-13 weeks). By 10 weeks, most major structures are formed resulting in a fetus. The second trimester (13-26 weeks) is associated with rapid fetal growth. Completion of maturation occurs in the third trimester (26-40 weeks) resulting in a term pregnancy between 37 and 42 weeks.

Diagnosis

Once pregnancy has been achieved, several laboratory tests are routinely performed to help ensure an optimal maternal and fetal outcome (Table 20–1). Most testing in pregnancy is performed on maternal serum because it is easy to obtain and provides minimal risk to the pregnancy, but maternal urine and amniotic fluid specimens may also be necessary. Of note, pregnancy has an effect on many laboratory tests, other than those used to diagnose and manage pregnancy, and these alterations should be considered when interpreting laboratory tests from pregnant women (Table 20–2).

hCG is one of the most commonly ordered tests in pregnancy. It is heterodimer composed of 2 nonidentical nonconvalently bound glycoprotein subunits, alpha and beta, that is synthesized by the trophoblasts of the placenta. Only the intact molecule is biologically active. A single gene for the alpha subunit of all 4 glycoprotein hormones (thyroid-stimulating hormone [TSH], luteinizing hormone [LH], follicle-stimulating hormone [FSH], and hCG) is found on chromosome 6. hCG stimulates the LH receptor on the corpus luteum to produce progesterone that helps to prevent pregnancy loss.

Detectable amounts of hCG (>2-5 IU/L, depending on the assay) are present in serum 8 to 11 days after conception. Qualitative urine hCG tests are usually sufficient for screening, but the detection limits of qualitative tests range from 20 to 50 IU/L, limiting their use to the time following a missed menstrual period or greater than 14 days after conception. As opposed to qualitative testing, quantitative testing offers sensitivity as low as 2 to 5 IU/L and serial measurements may be helpful to reveal problems in a pregnancy. In normal pregnancies, hCG doubles every 1.5 to 2 days for the first 8 weeks (Table 20–1) and peaks around 100,000 to 500,000 IU/L.

Blood and urine contain many hCG variants, including free subunits, and hyperglycosylated and nicked forms. After 5 weeks of gestation the predominant hCG form in urine is the hCG beta core fragment. In addition, the hCG glycosylation changes as the pregnancy progresses. Quantitative hCG immunoassays typically measure total hCG beta concentrations using 2 antibodies against different regions of the beta subunit. False-negative hCG results can be seen in early pregnancy or when hCG concentrations are very high, causing a hook effect. False-positive results can be caused by heterophilic antibodies and by advanced maternal age (from pituitary hCG).

Other additional routine testing in pregnancy is performed to screen for common and/or treatable pregnancy complications such as gestational diabetes (see Chapter 17), hemolytic disease of the newborn (see Chapters 7 and 12), and infection (see Chapter 5) (Table 20–1).

Description

Maternal serum screening can be performed to identify individuals who need further diagnostic evaluation for fetal anomalies such as neural tube defects (NTDs), trisomy 21/Down syndrome, and trisomy 18. NTDs result from failure of fusion of the neural plate and failure of complete covering by the 27th day post conception. The extent and location of neural tissue exposed indicates the severity of the defect (ie, anencephaly, meningomyelocele, and encephalocele). The result of a NTD is a direct communication of the amniotic fluid with fetal plasma proteins, and release of alpha-fetoprotein (AFP) into amniotic fluid and maternal serum. Rates of NTDs have decreased due to the addition of folic acid to grain, as well as initiation of recommendations for folic acid supplementation prior to conception. Trisomy 21 or Down syndrome is caused by an extra copy of chromosome 21 and is the most frequent chromosomal disorder among live-born children (1/600 to 1/800 live births). Risk factors for Down syndrome include advanced maternal age, the birth of a previously affected child, and balanced parental structural rearrangement of chromosome 21. Affected children suffer from mental retardation, hypotonia, congenital heart defects, and a flat facial profile. The main phenotypic features of trisomy 18 include hypertonia, prominent occiput, small mouth, micrognathia, short sternum, and horseshoe kidney.

Screening

An additional discussion of maternal screening for Down syndrome is found in Chapter 7.

Screening for NTDs is done at 15 to 22 weeks gestation by measurement of serum AFP that is expressed as a multiple of the median population (MoM). MoMs greater than 2 or 2.5 are considered abnormal and should be followed up by high-resolution ultrasound or measurement of amniotic fluid AFP and acetylcholinesterase.

Sequential serum screening is performed to screen for trisomies and combines first- and second-trimester testing. First-trimester testing is performed between 10 and 14 weeks and includes measurement of hCG and pregnancy-associated plasma protein-A (PAPP-A) as well as an ultrasound measurement for infant nuchal translucency (NT). Specific training is required of operators for determination of NT. This procedure is highly operator dependent. Free beta hCG testing is more accurate than intact hCG testing in the first trimester and is used instead of intact hCG for the first-trimester screen. PAPA-A is a protein produced by the placenta. Elevated hCG, decreased PAPP-A, and increased NT are seen in pregnancies affected by trisomy 21.