Primary Effect of Progesterone and Relaxin During Pregnancy
The correct answer is A: increasing levels of progesterone and relaxin during pregnancy soften the ligaments and muscles of the pelvic floor. This is the well-established primary physiological effect of these hormones, particularly relaxin, which targets connective tissues in the pelvis to facilitate childbirth.
Evidence for Pelvic Tissue Softening
Relaxin's primary established role is causing growth and increased elasticity of pubic joint cartilage and softening of pelvic structures during pregnancy. 1
- In humans and many mammalian species, there is strong evidence that relaxin causes growth and increased elasticity of the pubic symphysis during pregnancy 1
- The softening and increase in size of the cervix during the second half of pregnancy involves relaxin's effects on collagen, elastin, proteoglycans, and glycosaminoglycans that are responsible for the tensile properties of these tissues 1
- The vagina increases in size and softens during pregnancy, with good evidence from animal studies that these effects involve relaxin 1
Progesterone works synergistically with relaxin to enhance pelvic tissue softening. 2
- When progesterone is given in combination with relaxin, it markedly augments relaxin's effects on cervical softening and reduces the organization and density of cervical collagen fibers 2
- Progesterone alone has little effect on cervical softening, but acts in concert with relaxin to promote these changes 2
Why Other Options Are Incorrect
Option B (hemorrhoids) is not a primary hormonal effect:
- While hemorrhoids can occur during pregnancy, they result from mechanical pressure from the growing uterus on pelvic veins, not from direct hormonal action of progesterone or relaxin
Option C (milk production) is incorrect:
- Prolactin, not progesterone or relaxin, is the primary hormone that stimulates milk production
- While relaxin has roles in mammary nipple development in some species, this is species-dependent and not its primary function 1
- In humans, relaxin's role in mammary tissue is facilitatory rather than essential for milk production 1
Clinical Relevance
The pelvic ligament and tissue softening caused by these hormones prepares the birth canal for delivery but can also contribute to musculoskeletal symptoms during pregnancy 3. However, studies show that serum relaxin levels do not correlate with the severity of pelvic girdle pain, indicating the relationship is more complex than simple hormone concentration 4, 5.