Normally, the amounts of all steroid hormones increase as pregnancy progresses. The maternal unit responds to altered hormone levels even before the growing uterus is apparent. Serial hormone testing may be done to monitor rising levels of a particular hormone over a period of time. Decreasing levels indicate that the maternal–placental–fetal unit is not functioning normally. Biochemical analyses of several hormones can be used to monitor changes in the status of the maternal–fetal unit (see Chapters 3 and 6).
In early pregnancy, hCG in maternal blood provides evidence of a viable pregnancy. The hCG in maternal serum is measured as a sensitive pregnancy test (the hCG level increases 66%–100% every 48 hours during pregnancy). Also, it is used to monitor the success of in vitro fertilization or insemination, to diagnose trophoblastic tumor, to diagnose ectopic pregnancy (indicated by a decrease in hCG over a 48-hour period), and to screen for Down syndrome in pregnancy. For further discussion of pregnancy tests, see Chapter 6.
Together with prolactin and luteinizing hormone, hCG prolongs the life of the corpus luteum once the ovum is fertilized. It stimulates the ovary for the first 6–8 weeks of pregnancy, before placental synthesis of progesterone begins. Its function later in pregnancy (in maternal blood) is unknown.
PAPP-A, a circulating placental protein, has been shown to increase the stimulatory effects of placental insulin-like growth factors. Decreased serum levels in the maternal circulation in the first 10 weeks after conception are associated with uncomplicated full-term low birth weights. PAPP-A levels are detectable within 30 days after conception and slowly increase throughout the first 30 weeks of gestation. Maternal serum levels are 0.43 μg/L (12 pmol/L). Increased PAPP-A occurs with Down pregnancy.
Late in pregnancy, the levels of E3 and human placental lactogen (hPL) in maternal blood reflect fetal homeostasis. hPL is a protein hormone produced by the placenta. Testing of hPL evaluates only placental functioning. Blood testing of the patient usually begins after the 30th week and may be done weekly thereafter. A concentration of 1 μg/mL (46 nmol/L) hPL may be detected at 6–8 weeks of gestation. The level slowly increases throughout pregnancy and reaches 7 μg/mL (324 nmol/L) at term before abruptly dropping to zero after delivery. hPL functions primarily as a fail-safe mechanism to ensure nutrient supply to the fetus, for example, at times of maternal starvation. However, it does not appear to be required for a successful pregnancy outcome (see Chapter 6).