Researchers have studied uterine contractility, or peristalsis, for decades. It has been extensively observed playing an important role early in the reproductive process, helping sperm reach the oocyte in time for fertilization. Peristalsis also plays a role in prompting menstruation.
However, much less is understood about peristalsis after ovulation, especially its impact on embryo implantation following fertility treatments.
Overview of Uterine Contractility
Peristalsis is the continuous wave-like patterns of rhythmic contractions in the endometrium or sub-endometrial myometrium that shift according to the phase of the menstrual cycle. Abnormal contractility of the uterus — hyperperistalsis and dysperistalsis — may be present in the uterine junctional zone of patients with adenomyosis, endometriosis, unexplained infertility or recurrent embryo implantation failure. The electrophysiological reasons behind these contractions are not entirely clear.
These contractions are not unlike those that take place in other smooth muscle organs, such as the stomach. Stomach interstitial pacemaker cells create and synchronize gastrointestinal (GI) tract peristalsis. Each section of the GI tract has its own slow-wave frequency. Membrane potential changes cause short-term high and low excitability.
Similarly, interstitial Cajal-like cells (ICLCs) have been discovered in the uterus that express progesterone and estrogen receptors, according to research published in Biologia. Because ICLCs are near motor neurons and capillaries, slow-wave frequencies may be mediated by neurotransmitters, prostaglandins and hormones, adds a study published in Physiological Reviews. However, unlike ICLCs in the human GI tract, there is no evidence that myometrial ICLCs act as a pacemaker.
Previous Methods for Assessing Uterine Contractility
In the past, ultrasound-based hysterosalpingography (HSG) was used to obtain information about peristalsis. For example, a 1996 study published in Human Reproduction discussed how this method followed sperm through the genital tract and showed increased intensity and frequency of peristalsis as the follicular phase advanced.
Then, a 2003 study published in the Journal of Magnetic Resonance Imaging looked at the use of cine magnetic resonance imaging (MRI) with ultrafast imaging. The researchers reviewed 300 MRI images from 13 women over four menstrual cycles. They compared their findings to those of 59 other studies and determined this technique could accurately detect peristalsis during the early phases of a woman's menstrual cycle.
More recently, a 2020 study published in Revista Brasiliera de Ginecologia e Obstetricia examined the junctional zone with 3D ultrasound. Although this method observed endometrium thickness and disruptions — and the researchers recommended it as part of a routine assessment — the study acknowledged that 3D ultrasound is not yet a universally accepted technique for assessing peristalsis. The researchers reaffirmed that abnormal peristalsis results in lower pregnancy rates both in natural and assisted reproductive technology cycles.
The Future of 4D Ultrasound
The newest technique for monitoring peristalsis is combining 4D ultrasound video with an assessment of progesterone levels.
A 2021 study published in Fertility and Sterility looked at 197 patients with a history of recurrent embryo implantation failure using donor oocytes. Peristalsis in the uterus was accurately and easily viewed and recorded on video and assessed over the course of a six-minute clip. Interestingly, progesterone levels and uterine contractility were inversely related — lower levels resulted in hyperperistalsis.
4D ultrasound is a low-cost, accessible method of evaluating peristalsis, but additional, larger studies are needed before this method can become the gold standard.
