Ovulation is the biological process where a mature egg is released from one of the ovaries, making it available for fertilization. This cyclical event usually occurs about halfway through the menstrual cycle and is regulated by hormones. Many people who track their cycles note a consistent sensation of activity on only one side, leading them to question why their body appears to favor a single ovary.
The Typical Alternating Cycle
Reproductive biology expects the two ovaries to alternate in releasing an egg each month. This selection process is governed by hormones from the brain. Follicle-stimulating hormone (FSH) stimulates the growth of follicles, which produce estrogen. Eventually, one follicle becomes dominant, triggering a surge in luteinizing hormone (LH). This LH surge causes the dominant follicle to rupture and release the egg. In fertile women, this selection is random, with ovulation occurring on the left side in roughly half of the cycles and on the right side in the other half.
Why Sensation May Not Equal Location
The perception of unilateral ovulation is often linked to Mittelschmerz, or mid-cycle pain. This pain is typically one-sided, occurring around ovulation and lasting up to a day or two. The pain is caused by the irritation of the abdominal lining (peritoneum) by the fluid and blood released from the ruptured follicle. While the pain usually occurs on the side of the active ovary, nerve pathways can make the sensation misleading. Pain originating in one area can be felt in an adjacent region, known as referred pain. Furthermore, the fluid released can move within the pelvic cavity, irritating the lining away from the ovary’s exact location. Therefore, consistent pain on the left side does not definitively prove the right ovary is inactive.
Physiological Reasons for Ovarian Preference
If medical investigation confirms a genuine pattern of unilateral ovulation, specific anatomical or physiological factors are responsible.
Asymmetry in Blood Flow
One potential cause is an asymmetry in blood flow, where one ovary receives a more advantageous supply of oxygen and nutrients, giving its follicles a competitive edge. Slight differences in the neural signals (innervation) of the left and right ovary could also contribute to a functional imbalance.
Scar Tissue and Adhesions
Prior surgical history, such as an appendectomy or pelvic surgery, can create scar tissue or adhesions near one ovary. These adhesions may impede the function or mobility of that ovary, causing the other side to consistently take over.
Persistent Corpus Luteum
Another possible cause is the persistence of the corpus luteum from a previous cycle on the non-ovulating side. The corpus luteum produces progesterone, and if it fails to regress completely, the hormonal influence can suppress the development of a new dominant follicle on that same side in the subsequent cycle.
When to Consult a Healthcare Provider
For most people, persistent unilateral sensation is simply a quirk of pain signaling and does not require medical intervention. Observation is generally sufficient if you are under 35 and have been trying to conceive for less than a year. However, it is recommended to seek a consultation if you are over 35 and have been trying to get pregnant for six months without success, or if you are under 35 and have been trying for a year. You should also see a healthcare provider sooner if the pain is severe or prolonged, or if you have a known history of conditions that can affect the fallopian tubes or ovaries. Conditions like endometriosis, pelvic inflammatory disease (PID), or a previous ectopic pregnancy can cause scarring that may obstruct the fallopian tube on the non-ovulating side. In these cases, understanding which side is ovulating is important for assessing reproductive function and creating a personalized health plan.