There are three fundamental components to human reproduction that together determine a couple’s days of fertility – the sperm, egg, and well-estrogenized cervical mucus.
While discussions about fertility often focus on sperm and the egg, cervical mucus deserves just as much attention.
Beginning in the pre-ovulatory phase of a woman’s cycle, estrogen produced by the developing follicle stimulates hormonally-responsive glands within the female cervix to produce a unique type of fertile mucus.
This well-estrogenized mucus is referred to as the “biological valve” of fertility, because its physical properties provide a network of intricate channels specifically designed for sperm transportation.
Like lanes on a highway, fertile-type cervical mucus guides sperm toward the egg. When it is no longer produced, those pathways effectively close—along with a couple’s ability to conceive.
Research shows that when a woman produces little or no fertile-type cervical mucus, the chances of conception in a given cycle decrease. Conversely, when a couple can accurately identify and time intercourse during peak fertility days, their chances of achieving pregnancy in that cycle increase.
Given the important role fertile-type cervical mucus plays in reproduction, researchers have sought to understand the biological mechanisms behind its production. This work has identified a key link between a small signaling molecule—nitric oxide (NO)—and the production of cervical mucus during the female cycle.
The link to NO comes from evidence that the female nervous system helps regulate cervical mucus production through nerves located within the cervix itself.
NO acts as a key cellular messenger in the body, supporting a range of physiological functions, including nerve signaling, inflammation, immune responses, and vascular regulation.
Researchers propose that rising ovarian hormone levels around ovulation stimulate local NO production in the cervix. As NO levels increase, this signaling molecule activates nearby nerves, which in turn trigger hormone-responsive glands to produce cervical mucus. This fertile-type mucus effectively “opens” the reproductive pathway, allowing sperm to travel toward the egg and making conception possible.
To explore this connection further, a study by Mörlin et al. examined the effects of nitroglycerin on cervical mucus production. Nitroglycerin is a NO donor—a medication that is converted into NO in the body. The researchers found that administering nitroglycerin early in the female cycle led to a significant increase in cervical mucus production.
Based on these findings, the authors concluded that fertile-type mucus production is likely influenced by the nervous system, with NO serving as a key biological mediator in the process.
Supporting this idea, other studies have shown that cervical mucus production decreases when women take medications that inhibit certain chemical messengers essential for normal nervous system function.
In humans, NO is produced naturally through two main pathways: the breakdown of dietary nitrates—found in certain foods—and the conversion of the amino acid L-arginine, which is obtained from both plant- and animal-based sources.
From a dietary perspective, vegetables are the primary source of nitrate-derived NO. Foods especially rich in nitrates include leafy greens and root vegetables such as beets, cress, lettuce, spinach, arugula (rucola), and celery.
Together, these findings help clarify the biological processes behind fertile-type cervical mucus production, highlighting NO as a key molecule involved in activating the “biological valve” that supports human fertility.
Recognizing the role of NO may be particularly important for couples experiencing unexplained infertility or for those who observe little or no fertile-type cervical mucus.
References:
[1] Hilgers TW, Daly KD, Prebil AM, Hilgers SK. Cumulative pregnancy rates in patients with apparently normal fertility and fertility-focused intercourse. J Reprod Med. 1992 Oct;37(10):864-6. PMID: 1479570.
[2] Mörlin B, Hammarström M. Nitric oxide increases endocervical secretion at the ovulatory phase in the female. Acta Obstet Gynecol Scand. 2005 Sep;84(9):883-6. doi: 10.1111/j.0001-6349.2005.00752.x. PMID: 16097981.
[3] Olas B. The Cardioprotective Role of Nitrate-Rich Vegetables. Foods. 2024 Feb 24;13(5):691. doi: 10.3390/foods13050691. PMID: 38472804; PMCID: PMC10931520.
[4] Thijssen A, Meier A, Panis K, Ombelet W. ‘Fertility Awareness-Based Methods’ and subfertility: a systematic review. Facts Views Vis Obgyn. 2014;6(3):113-23. PMID: 25374654; PMCID: PMC4216977.
[5] Tuteja N, Chandra M, Tuteja R, Misra MK. Nitric Oxide as a Unique Bioactive Signaling Messenger in Physiology and Pathophysiology. J Biomed Biotechnol. 2004;2004(4):227-237. doi: 10.1155/S1110724304402034. PMID: 15467163; PMCID: PMC555773.
