The 40 weeks of an average pregnancy are usually described in terms of three trimesters, each approximately 13 weeks. During the first trimester, as the placenta develops, it gradually becomes the endocrine organ of pregnancy. Estrogen maintains the pregnancy, promotes fetal viability, and stimulates tissue growth in the mother and developing fetus. Progesterone prevents new ovarian follicles from developing and suppresses uterine contractility until labor. The hormone relaxin has beneficial effects on the endometrium responsible for establishment of pregnancy. This hormone also stimulates endometrial decidualization—a process that results in significant changes to cells of the endometrium in preparation for and during pregnancy. This includes morphological and functional changes to endometrial cells, the presence of white blood cells, and vascular changes to maternal arteries (Goldsmith and Weiss 2009).
Three trimesters of pregnancy and fetal milestones.
There is general consensus in the literature that joint laxity increases during pregnancy and that this is essential for adapting to the growing foetus and preparing for childbirth. Links have also been made with various musculoskeletal disorders. Metacarpophalangeal (relating to the joints at the base of the fingers) and generalized laxity is thought to increase considerably during the second trimester of pregnancy (Cherni et al. 2019). Around 45 percent of all pregnant women and 25 percent of all postpartum women suffer from pregnancy-related pelvic girdle pain and pregnancy-related low back pain (Wu et al. 2004). According to Carvalho and colleagues (2017), low back pain is also more frequent in the second trimester of pregnancy. Pain in the hand and wrist is the second most prevalent musculoskeletal symptom during pregnancy (Nygaard et al. 1989).
However, it is not so clear what role the pregnancy hormones play in this. A review by Dehghan and colleagues (2014) on the effect of relaxin on the musculoskeletal system highlighted conflicting evidence for the role that relaxin plays in increased joint laxity. The review stated that the role of this hormone on the human pubic symphysis (the junction between the two hip bones at the front of the pelvis) is unknown. A systematic review by Aldabe and colleagues (2012) stated that a direct relationship between high levels of relaxin and increased pelvic mobility or peripheral joint mobility in pregnant women has not been shown. Marnach and colleagues (2003) stated that while peripheral joint laxity increases during pregnancy, these changes do not correlate well with maternal estrogen, progesterone, or relaxin levels.
Maybe in the future with more research we will become clearer on the underlying processes that cause increased joint laxity during pregnancy.
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References:
Aldabe, D., D. Ribeiro, S. Milosavljevic, and M. Dawn Bussey. 2012. “Pregnancy-Related Pelvic Girdle Pain and Its Relationship with Relaxin Levels During Pregnancy: A Systematic Review.” European Spine Journal 21 (9): 1769-1776.
Carvalho, M., L. Lima, C. de Lira Terceiro, D. Pinto, M. Silva, G. Cozer, and T. Couceiro. 2017. “Low Back Pain During Pregnancy.” Revista Brasileira de Anestesiologia 67 (3): 266-270.
Cherni, Y., D. Desseauve, A. Decatoire, N. Veit-Rubinc, M. Begon, F. Pierre, and L. Fradet. 2019. “Evaluation of Ligament Laxity During Pregnancy.” Journal of Gynecology Obstetrics and Human Reproduction 48 (5): 351-357.
Dehghan, F., B. Haerian, S. Muniandy, A. Yusof, J. Dragoo, and N. Salleh. 2014. “The Effect of Relaxin on the Musculoskeletal System.” Scandinavian Journal of Medicine and Science in Sports 24 (4): e220-e229.
Goldsmith, L., and G. Weiss. 2009. “Relaxin in Human Pregnancy.” Annals of the New York Academy of Sciences 1160:130-135.
Marnach, M., K. Ramin, P. Ramsey, S. Song, J. Stensland, and K. An. 2003. “Characterization of the Relationship Between Joint Laxity and Maternal Hormones in Pregnancy.” Obstetrics and Gynecology 101 (2): 331-335.
Nygaard, I., C. Saltzman, M. Whitehouse, and F. Hankin. 1989. “Hand Problems in Pregnancy.” American Family Physician 39:123-126.
Wu, W., O. Meijer, K. Uegaki, J. Mens, J. van Dieën, P. Wuisman, and H. Ostgaard. 2004. “Pregnancy-Related Pelvic Girdle Pain (PGP), I: Terminology, Clinical Presentation, and Prevalence.” European Spine Journal13:575-589.