There once was an OBGYN who had an ultrasound scanner in his office, some extra time, and an inventive curiosity. He wanted to know what uteri did all day long. He thought that being a smooth muscle organ, it probably would contract from time to time, but he wasn’t sure.
So he persuaded some of his patients to lie very still while he scanned them continuously, while he watched the monitor screen carefully for any signs of contractions. He didn’t see anything.
So he called on his friend, another OBGYN, to watch the videotapes of his scanning to confirm that nothing was happening on them. His friend agreed, but found that watching these tapes was very tedious, so he hit the fast forward button. To his surprise, the uterus that appeared to be at rest at normal speed, was visibly contracting when he sped up the tape.
Not only was the uterus contracting constantly, it was contracting in a very consistent pattern. The contractions mostly started at the fundus and squeezed down and out the cervix, just like it would do if it were trying to deliver a baby.
With further study, several additional things were discovered that I think provide insight into several of the many unanswered questions in OBGYN.
First, this down and out contraction pattern was quite evident during menses and also leading up to ovulation.
But with ovulation, the contraction pattern reversed, with most contractions starting near the cervix and squeezing up and out the fallopian tube.
Well, this is very important in terms of sperm transport. You see, sperm are microscopic creatures who couldn’t swim one centimeter in an entire sperm lifetime. So how is it that sperm deposited around the cervix will appear at the fimbriated end of the fallopian tube 10 minutes later? It’s because they’re not swimming that far…they’re being propelled by the uterine muscular squeeze that picks them up at the cervix and delivers them deep into the fallopian tube, where the villous membrane further propels them to their target, the ovum.
This may also be important in some cases of infertility. You see, not everyone followed this pattern of up and out squeezes during ovulation. For some women, the contractions were not coordinated, or the uterine squeezes were from right to left or left to right, not a pattern that would be expected to contribute to sperm transport.
This may also be important in the pathogenesis of endometriosis. If, during menses, instead of most of the contractions being in a down and out pattern, many are starting near the cervix and squeezing up and out, I would expect more than an average amount of menstrual products to be propelled through the fallopian tubes and into the abdomen, where some may successfully implant. It takes no great imagination to connect these dots and say that if uterine contraction patterns are abnormal, that may increase the likelihood both of endometriosis and of infertility.
Following ovulation, the uterine contractions largely stop, and the uterus remains relatively relaxed until shortly before menstruation. This period of quiescence coincides with the ovaries production of progesterone. Progesterone is known to have smooth muscle relaxing properties. It’s absence immediately after conception is an established cause of infertility, and abnormally low levels are known as a “luteal phase defect” and are believed by many to lead to loss of an early pregnancy. The mechanism by which lack of progesterone leads to these early losses is not known, but one contributing factor may be lack of progesterone-inspired uterine contractile quiescence that is normally seen during the luteal phase.
The progesterone effect on uterine contractions may also play a role in hormone-based contraceptives. These contraceptives are thought to exert their effects in a number of overlapping ways, among them ovulation suppression, and a change in cervical mucous making it less permeable to sperm. An additional factor may be the impact of progestins on uterine contractility, interfering with the ovulatory surge in cervix to fallopian tube uterine contractions, thus interfering with sperm transport to the fallopian tube.
So what does a uterus do all day long? It contracts in ways that are beneficial to preserving health and our species.