Fetal monitoring and prenatal diagnosis

Biochemical tests have limited value in monitoring fetal development, but some components of maternal blood and urine and amniotic fluid may be measured to give evidence of pathology.

HCG

Human chorionic gonadotrophin (HCG) is a glycoprotein produced by the chorionic cells of the developing embryo that is detectable by sensitive assays within days of conception. Measurement of HCG is used to confirm pregnancy, and forms the basis of pregnancy tests (p. 9). The protein’s rapid rate of synthesis in early pregnancy provides systemic evidence of the blastocyst 24 hours after implantation. HCG continues to be secreted by the developing placenta, and serum and urine concentrations increase during the first 9 weeks of pregnancy, then decline gradually until the third trimester (Fig 75.1). The function of HCG is to maintain the activity of the corpus luteum sustaining progesterone synthesis. Measurement of HCG is also of value in:

Fig 75.1 HCG concentration in maternal blood in early pregnancy.

Assessing fetal viability in threatened abortion.

Detecting ectopic pregnancy. HCG fails to rise at the expected rate. In the first trimester of a normal pregnancy it approximately doubles every 48 hours.

Detecting and monitoring hydatidiform mole and choriocarcinoma. HCG may be used as a tumour marker for diagnosis and monitoring of these trophoblastic malignancies (pp. 140–141).

Fetoplacental function

Human placental lactogen (HPL) is synthesized by the placenta. The HPL concentration in maternal blood increases during pregnancy until term. It can be used to monitor placental function.

Oestriol is a steroid synthesized by the combined action of enzymes in the fetus and placenta. Its concentration in maternal blood increases throughout pregnancy, and it can be assayed in maternal urine or blood. Oestriol was at one time commonly used to monitor fetoplacental function.

These two biochemical tests have been superseded by physical investigations such as ultrasound and cardiotocography.

Prenatal diagnostic techniques fall into two groups: invasive and non-invasive (Table 75.1). Prenatal diagnosis may be required because of increased risk of inherited disease. Neural tube defects cannot usually be predicted by family history, and pregnant women may be offered a screening test to detect these disorders. For further details on antenatal screening see pp. 154–155.

Table 75.1

Techniques for prenatal diagnosis

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