Which hormone is primarily responsible for nausea and vomiting in early pregnancy?

Urinary frequency in pregnancyoccurs due to early uterineenlargement and increased renalchanges that affect bladder capacity and function. By 6 weeks gestation, the uterus rises out of the pelvis, exerting pressureon the bladder, while renal plasma flow increases by 50–80% and glomerular filtration rate rises by 40–50% above the non-pregnant baseline of 90–120 mL/min/1.73 m². Increased filtrationalso contributes to physiologic glycosuria and mild polyuria. These adaptations are normal and help accommodate maternal and fetal metabolic demands.

Rationale for correct answers

2.Pressure from the enlarging uterus on the bladder is the main cause of urinary frequency in the first trimester. The uterus grows rapidly, and by 12 weeks gestation it is large enough to exert mechanical compression on the bladder, reducing capacity and increasing micturition episodes.

Rationale for incorrect answers

1.Decreased renal blood flow is not present in early pregnancy. Instead, renal plasma flow increases by 50–80% due to systemic vasodilation mediated by progesterone and relaxin. Increased perfusion, not decreased, characterizes renal physiology in early gestation.

3.Reduced glomerular filtration rate is not correct because GFR increases by 40–50% above baseline in pregnancy, reaching values around 150–180 mL/min/1.73 m². This rise leads to increased clearance of urea, creatinine, and electrolytes. Normal non-pregnant creatinine is 0.6–1.1 mg/dL, but in pregnancy levels drop to 0.4–0.8 mg/dL due to elevated GFR.

4.An increase in antidiuretic hormone does not occur in early pregnancy. Instead, there is a mild resetting of the osmotic threshold for thirst and ADH secretion, but baseline ADH concentrations remain unchanged. Plasma osmolality decreases by about 10 mmol/kg, yet this does not directly cause urinary frequency.

Take home points

• Urinary frequency in early pregnancy is primarily mechanical, due to uterine pressure on the bladder.
• Renal plasma flow and GFR increase significantly in pregnancy, not decrease.
• Normal creatinine levels are lower in pregnancy due to increased filtration.
• ADH secretion is unchanged; only the osmotic threshold for thirst is reset.

Human chorionic gonadotropin (hCG)is a glycoprotein hormone produced by trophoblasts after implantation. It maintains the corpus luteum, stimulates progesteronesecretion, and peaks at 8–12 weeks gestation, reaching levels of 50,000–100,000 mIU/mL (normal non-pregnant <5 mIU/mL). High hCGconcentrations stimulate the chemoreceptor trigger zone, causing nauseaand vomiting. Elevated levels occur in molar pregnancy and multiple gestations.

Rationale for correct answers

2.hCG is the primary hormone responsible for nausea and vomiting in early pregnancy. Its peak around the end of the first trimester coincides with maximal morning sickness severity. Its stimulation of the chemoreceptor trigger zone and delayed gastric emptying explains the symptoms described.

Rationale for incorrect answers

1.Progesterone causes smooth muscle relaxation, decreased lower esophageal sphincter tone, and reduced gastrointestinal motility, which can contribute to bloating and constipation, not the primary mechanism for nausea and vomiting.

3.Estrogen is elevated in pregnancy and may have some role in enhancing sensitivity to odors, but its primary functions include uterine growth, increased blood flow, and breast tissue development. It does not directly stimulate the vomiting center.

4.Relaxin is secreted by the corpus luteum and placenta and functions mainly to relax pelvic ligaments and soften the cervix in preparation for delivery. It does not act on the gastrointestinal tract or central emetic pathways.

Take home points

• hCG is the main hormone responsible for nausea and vomiting in early pregnancy.
• Progesterone mainly causes constipation and reflux due to smooth muscle relaxation.
• Estrogen contributes to vascular changes and uterine growth, not emesis.
• Relaxin softens pelvic ligaments and cervix, unrelated to nausea.