If you've been involved in a pregnancy a birth experience you've heard lots of talk of the cervix - that is the end of the uterus that closes to hold the baby in and opens to allow the baby to be born. Dilation of the cervix is one of the main measurements used before and during labor to measure progress (though dilation is not as linear and predictable as we like to think). Effacement is also a measure of the cervix that is often checked at the end of pregnancy and through labor, along with the texture, and position of the cervix. These last two tend to be measurements that are paid more attention in late pregnancy, but stop being discussed in labor because the cervix softens and moves forward to prepare for labor, and often completes much of this before labor begins or in early labor.
What we often don't talk about is the cervix's role in menstruation. Because the cervix is in essence the door to the uterus, the cervix has to open slightly to allow menstruation to occur. It also does open around ovulation to allow the possibility of insemination.
While we often talk about menstrual cramps as a uterine phenomenon, it is actually more likely that many cases of dysmenorrhea (painful periods) are due to the cervix. In a 2021 study by Xholli et al, they found a correlation between cervical hardness and dysmenorrhea. Obviously there are other causes of dysmenorrhea, but this can reorient how many of us understand what is happening in our bodies at this time. For example many people have the experience of cramps that are in the first day(s) of menstruation and as the cramps ease or cease, there is an experience of a higher volume of blood being expelled. If we think of the course of the cramps as related to the cervical opening this makes perfect sense - as the cervix dilates there is sensation and cramping, and once it is open there is more space for menstrual blood to flow. This also helps explain why early labor sensations and menstrual cramping are similar, with labor sensations getting stronger as they go much past the minor dilation of menstruation (less than 1cm), to the ~5cm that is used as an indication of the start of active labor, and then to the 10cm that makes enough space for the baby to be born.
Another thing we often don't talk about is the make up of cervical tissue. The cervix has muscle tissue (Vink, et al, 2016). This means the cervix is contractile. The cervix has circular muscle fibers (much like a sphincter), which help it remain closed, these are primarily on the internal surface of the cervix. There are also longitudinal muscle fibers, which help the cervix open. There was also a study in the 1950s that found that the cervix and uterus contract independently from one another in response to contractile agonists (that is chemicals that stimulate contractions) (Schild, et al, 1951). This means we may benefit from considering the cervix and uterus actions separately and evaluating what each is doing and if there is dysfunction, which one is at the root.
Any muscle, including smooth muscle can be hyper- or hypo-tonic (tight or lax). So what are the implications of cervical muscle dysfunction? One thought is that the muscles being too tight could make the opening process harder or cause more sensation with the opening. Something that was proposed in the Schild study is that weakness in the muscles of the internal cervical os (that is the inner opening that faces the uterus) is a cause of premature labor, because the muscle are unable to perform a sphincter-like function.
Part II of this post will explore some potential solutions for different circumstances. Stay tuned.
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