Which hormone is primarily responsible for relaxing smooth muscles throughout the body during pregnancy, leading to effects such as heartburn and constipation?

Human chorionic gonadotropin (hCG)is a glycoproteinhormone secreted by the syncytiotrophoblastduring early pregnancy. Its main function is to support the corpus luteum, ensuring continuous secretion of progesterone and estrogen until placental steroidogenesis is sufficient. Serum hCG normally doubles every 48–72 hours in early gestation and peaks at 100,000 IU/L around 10 weeks.

Rationale for correct answer

2.hCG maintains the corpus luteum in the first trimester. This prevents luteal regression and ensures sustained progesterone production, which maintains endometrial decidualization and prevents menstruation. The question stem asking about the primary role makes this the correct response.

Rationale for incorrect answers

1.Promoting insulin resistance is primarily mediated by human placental lactogen (hPL), not hCG. hPL increases maternal lipolysis and reduces insulin sensitivity, ensuring more glucose is available for the fetus. hCG has no direct role in glucose regulation.

3.hCG has mild thyrotropic activity due to structural similarity with thyroid-stimulating hormone (TSH), which may increase thyroid hormone levels in some women, but this is not its primary role. Normal TSH range is 0.4–4.0 mIU/L, and clinically significant changes are uncommon in healthy pregnancy.

4.Relaxation of smooth muscles, such as the uterus and gastrointestinal tract, is primarily mediated by progesterone, not hCG. Progesterone reduces uterine contractility and slows intestinal motility, leading to symptoms such as constipation and reflux. hCG does not mediate smooth muscle relaxation.

Take home points

• hCG is secreted by syncytiotrophoblasts and maintains the corpus luteum during early pregnancy.

• hPL regulates maternal glucose metabolism and causes insulin resistance, not hCG.

• Progesterone maintains uterine quiescence by relaxing smooth muscle.

• hCG has weak TSH-like activity but this is not its primary physiological role.

Human placental lactogen (hPL)is a peptide hormonesecreted by the syncytiotrophoblast. It modifies maternal metabolismby increasing insulin resistance, thereby reducing maternal glucose uptake and ensuring fetal glucose supply. hPL also promotes lipolysis, increasing free fatty acids for maternal energy use. Normal maternal serum hPL levels rise progressively to 5–7 mg/L at term.

Rationale for correct answer

2.hPL promotes insulin resistance by antagonizing insulin action in maternal tissues. This spares glucose for the fetus while shifting maternal metabolism toward lipolysis. The physiologic insulin resistance peaks in the second and third trimesters, helping maintain fetal growth.

Rationale for incorrect answers

1.Human chorionic gonadotropin (hCG) maintains the corpus luteum in early pregnancy and supports progesterone secretion. It does not influence insulin sensitivity or glucose regulation.

3.Estrogen promotes uterine growth, increases uteroplacental blood flow, and enhances myometrial oxytocin receptor expression. It does not play a significant role in maternal insulin resistance.

4.Progesterone relaxes smooth muscles, maintaining uterine quiescence and reducing gastrointestinal motility. While it indirectly affects glucose metabolism, it is not the primary hormone responsible for insulin resistance in pregnancy.

Take home points

• hPL is the main hormone that promotes insulin resistance in pregnancy.

• Insulin resistance spares glucose for the fetus and shifts maternal metabolism to fat utilization.

• Excessive insulin resistance may contribute to gestational diabetes mellitus.

• Estrogen, progesterone, and hCG serve other reproductive roles without directly altering insulin sensitivity.