Ovulation 101: What is it & how does it work?

Top things to know:

• Your eggs live in sacs in your ovaries, called follicles
• At any given time, there are follicles at all stages of development in your ovaries
• Follicles need the right conditions to grow and release their egg: your environment, health, and behaviors all play a role
• The processes of and around ovulation provides hormones that are crucial for your long-term health

What is ovulation?

Ovulation is the release of an egg from your ovary, into your fallopian tube. It typically happens about 13–15 days before the start of each period (1). Like your period, the timing of ovulation can vary cycle-to-cycle, and you may have the odd cycle where you don’t ovulate at all.

Understanding the basics of how ovulation works can be a powerful tool for your health. It’s common that people are introduced to the topic only after they have trouble becoming pregnant. But having a grasp on the process can give you insight into more than fertility. You’ll better understand any hormonally-influenced changes to your body around that time, and learn what factors might affect the timing of your ovulation (like stress), and why.

In the west, we ovulate roughly 400 times throughout our lifespan (2). This number is influenced by the use of contraceptives (many of which block ovulation), time spent pregnant and breastfeeding, and any behaviors or health conditions that affect the reproductive hormones (e.g. eating disorders. PCOS). Prehistorically, women would have ovulated less than half as often (3).

A lot goes into every ovulation. And if conditions aren’t right, it won’t happen. The development and release of an egg each cycle occurs in response to the intricate ups and downs of your reproductive hormones. Ovulation (and the menstrual cycle as a whole) is impacted by energetic, nutritional, emotional, and socioeconomic stressors. Short term factors like jet lag, seasonal changes, stress and smoking can have an effect, as well as longer term factors like PCOS and thyroid disorders (4–8).

How it works: the basics

Tiny eggs develop in sacs called follicles in your ovaries. Follicles develop for several cycles before they are ready to release their egg, (estimated at >175 days, or >~6 cycles) (9, 10). At any given time, there are follicles at several stages of development in your ovaries (10).

Early scientists once mistook the follicle for the egg itself.

Follicles undergo incredible changes leading up to ovulation, developing a plethora of parts and layers, each with their own functions. Most follicles, though, will never reach ovulation, dying off at different phases of development or pre-development.

At the beginning of each menstrual cycle, a few (~10) developing follicles are considered candidates for that cycle’s ovulation (11). By about midway through the follicular phase, one follicle becomes dominant.

Just like a superstar athlete being selected from the pack, all resources then go to preparing that single follicle, and the other potentials die off.

When the follicle is ready, it releases its egg. The egg travels out of the ovary and is taken up by the fallopian tube (the two aren’t actually attached — think of the end of the fallopian tube collecting the egg from the abdominal cavity). After its release, the egg has about 12–24 hours to be fertilized by sperm in the fallopian tube. If the egg isn’t fertilized within this short window, it begins to degrade. If it is fertilized, it travels to the uterus over the following 6–12 days, to possibly implant for pregnancy (12, 13).

All of these events are driven forward by the cyclical changes in your reproductive hormones. Hormones control the selection and development of your follicles, the release of each egg, and the preparation of your uterus for possible implantation.

This first part of the cycle is called the follicular phase (now you know where it gets its name).

Story time: Your brain, hormones, and ovulation

The brain-ovarian connection that controls follicle development and ovulation is called the Hypothalamic-Pituitary Axis, or HPO Axis.

Before ovulation: The brain produces continuous bursts of follicle stimulating hormone, or FSH, throughout your cycle. FSH does exactly what it’s named for — it stimulates follicles to develop (2). As follicles grow, they produce estrogen.

As a dominant follicle is selected and grows (days 6–9 of the cycle), estrogen begins to spike (10). A follicle becomes dominant at about 10mm in diameter, and typically grows to be about two centimeters in diameter (and up to about 3.3cm) (14–17). That’s about the size of a grape, or a hazelnut with its shell on.

Ready for release: When the amount of estrogen reaches its upper threshold, the egg is ready for release. The brain then produces a surge of luteinizing hormone (LH), triggering ovulation. The release of the egg from the follicle and ovary happens about 24 later (10–12 hours after LH peaks) (13, 17). The follicle uses enzymes to degrade its own wall and form an opening, allowing the release of the tiny egg from its center (18).

At the end of the fallopian tube, a finger-like structure swells with blood to grab and usher the egg in. Meanwhile, the egg has been undergoing its own changes to prepare for possible fertilization.

The pre-ovulatory follicle is the primary source of estrogen in the body. The dominant follicle is the source of >90% of the estrogen production in the pre-ovulatory period (19).

Take a look at the hormones graph. You’ll see estrogen, in blue, starts low and begins to spike midway through the follicular phase.

After ovulation: The egg may be on its way, but the follicle’s job is far from over. Luteinizing hormone transforms the large estrogen-making follicle into a progesterone-making machine (10). This new progesterone-producer (which also produces estrogen) is called the corpus luteum, or “yellow body” in latin (because it’s yellowish in color). A new corpus luteum is made every cycle in which ovulation occurs. If ovulation doesn’t happen, you won’t get that surge in progesterone at all.

If the egg isn’t fertilized by sperm and then implanted in the uterine lining (after traveling down the fallopian tube, which takes several days), the corpus luteum begins to degrade, and hormone levels drop, triggering your period (20). If a pregnancy does happen, the corpus luteum provides enough progesterone for your pregnancy to develop, until the placenta can take over (21).

Look at the hormones just after ovulation. Levels of estrogen drop slightly, and then progesterone (and estrogen) start to rise. If there is no hormonal signal that a pregnancy has begun, they begin to drop again midway through the luteal phase, eventually triggering the period.

Your ovulation is not a clock

Any factors that influence the hormonal pulsing in your brain can influence your ovulation. Environmental and internal factors like stress, diet and exercise changes can lead your ovulation to happen slightly earlier, or later, or not at all. Your period may then come earlier or later as well, and be lighter or heavier.

Your follicular phase is considered “plastic” as compared to your luteal phase. That means it can commonly change in length, from cycle-to-cycle. If you know the length of your typical luteal phase (most often 13–15 days) you can count backwards to get an idea of when you ovulated. Changes in the length of your cycle are usually pinpointed in the follicular phase — the time it takes a follicle to reach the point of ovulation.

It’s common not to ovulate on a regular basis when you first start menstruating. It’s also common to have irregular ovulation just after pregnancy and breastfeeding, and during the years approaching menopause.

Why it matters

Not ovulating every once in awhile may not be a concern, but if it becomes common, or if you stop ovulating altogether (and aren’t getting hormones in another way), serious health concerns can arise as a result.

The processes of and around ovulation provides your body with much-needed levels of estrogen and progesterone — hormones that play a role well beyond fertility. They impact your bone density, heart health, metabolism, sleep quality, mental health, and beyond. Anovulation in the fertile years is associated with osteoporosis, cardiovascular disease, and certain cancers later in life (22–25). Athletes with menstrual dysfunctions, for example, are significantly more likely to suffer from stress fractures (26).

How do I know if I’m ovulating?

As an adult, you are probably ovulating most of the time if your cycle is generally within range (that’s 24–38 days for adults, with fewer than 7–9 variance cycle-to-cycle, and a menstrual period of 2–7 days) (27). Cycles that are consistently outside of those ranges (they are long, short, or very irregular) — can be an indication of anovulation (or another health issue), and are a reason to talk to your healthcare provider.

To know if you’re ovulating (and when in your cycle it happens), you might try:

• Tracking your cycle length and regularity in Clue
• Using ovulation urine tests, bought at your pharmacy
• Tracking your physical symptoms for a few cycles, including your basal body temperature and cervical fluid
• Having your healthcare provider check your hormonal profile (by testing a sample of your blood, taken during your mid-luteal phase)

References

1. Lenton EA, LANDGREN B, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle: identification of the short luteal phase. BJOG: An International Journal of Obstetrics & Gynaecology. 1984 Jul 1;91(7):685–9.
2. Hillier SG. Current concepts of the roles of follicle stimulating hormone and luteinizing hormone in folliculogenesis. Human Reproduction. 1994 Feb 1;9(2):188–91.
3. Eston, SB, Pike, MC, Short, RV et al. Women’s reproductive cancers in evolutionary context.. Quart Rev Biol. 1994; 69: 353–367
4. Mahoney MM. Shift work, jet lag, and female reproduction. International journal of endocrinology. 2010 Mar 8;2010.
5. Vitzthum VJ, Thornburg J, Spielvogel H (2009) Seasonal modulation of reproductive effort during early pregnancy in humans. Am J Hum Biol 21: 548–558. Pmid:19402035
6. Kalantaridou, S. N., Makrigiannakis, A., Zoumakis, E., & Chrousos, G. P. (2004). Stress and the female reproductive system. Journal of Reproductive Immunology, 62(1), 61–68.
7. Windham GC, Elkin EP, Swan SH, Waller KO, Fenster L. Cigarette smoking and effects on menstrual function. Obstetrics & Gynecology. 1999 Jan 1;93(1):59–65.
8. Krassas GE, Poppe K, Glinoer D. Thyroid function and human reproductive health. Endocr Rev. 2010;31(5):702–55.
9. Early embryonic development: Moore KL, Persaud TV, Torchia MG. The developing human: clinically oriented embryology. Elsevier Health Sciences; 2015 Apr 6.
10. Baerwald AR, Adams GP, Pierson RA. Ovarian antral folliculogenesis during the human menstrual cycle: a review. Human reproduction update. 2011 Nov 8;18(1):73–91.
11. van Disseldorp J, Lambalk CB, Kwee J, Looman CW, Eijkemans MJ, Fauser BC, Broekmans FJ. Comparison of inter-and intra-cycle variability of anti-Müllerian hormone and antral follicle counts. Human reproduction. 2009 Oct 19;25(1):221–7.
12. Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. New England Journal of Medicine. 1999 Jun 10;340(23):1796–9
13. Cunningham FG. Williams Obstetrics 24th edition McGraw-Hill Education.
14. Lujan ME, Kepley AL, Chizen DR, Lehotay DC, Pierson RA. Development of morphologically dominant follicles is associated with fewer metabolic disturbances in amenorrheic women with polycystic ovary syndrome: a pilot study. Ultrasound in Obstetrics & Gynecology. 2010 Dec 1;36(6):759–66.
15. Macklon NS, Fauser BC. Regulation of follicle development and novel approaches to ovarian stimulation for IVF. Human Reproduction Update. 2000 Jul 1;6(4):307–12.
16. Ojengbede OA, Abidogun KA, Fatukasi UI. Ultrasound monitoring of ovarian follicular growth during spontaneous cycles in Nigerian women. African journal of medicine and medical sciences. 1992 Dec;21(2):57–61.
17. Kerin JF, Edmonds DK, Warnes GM, Cox LW, Seamark RF, Matthews CD, Young GB, Baird DT. Morphological and functional relations of Graafian follicle growth to ovulation in women using ultrasonic, laparoscopic and biochemical measurements. BJOG: An International Journal of Obstetrics & Gynaecology. 1981 Feb 1;88(2):81–90.
18. Russell DL, Robker RL. Molecular mechanisms of ovulation: co-ordination through the cumulus complex. Human reproduction update. 2007 Jan 22;13(3):289–312.
19. BAIRD DT, FRASER IS. Blood production and ovarian secretion rates of estradiol-17β and estrone in women throughout the menstrual cycle. The Journal of Clinical Endocrinology & Metabolism. 1974 Jun 1;38(6):1009–17.
20. Poyser NL. The control of prostaglandin production by the endometrium in relation to luteolysis and menstruation. Prostaglandins, leukotrienes and essential fatty acids. 1995 Sep 1;53(3):147–95.
21. Csapo AI, Pulkkinen M. INDISPENSABILITY OF THE HUMAN CORPUS LUTEUM IN THE MAINTENANCE OF EARLY PREGNANCY LUTEECTOMY EVIDENCE. Obstetrical & gynecological survey. 1978 Feb 1;33(2):69–81.
22. Li D, Hitchcock CL, Barr SI, Yu T, Prior JC (2014) Negative Spinal Bone Mineral Density Changes and Subclinical Ovulatory Disturbances — Prospective Data in Healthy Premenopausal Women With Regular Menstrual Cycles. Epidemiol Rev 36: 147. mxt012 [pii];
23. Gorgels WJ, Graaf Y, Blankenstein MA, Collette HJ, Erkelens DW, Banga JD (1997) Urinary sex hormone excretions in premenopausal women and coronary heart disease risk: a nested case-referent study in the DOM-cohort. J Clin Epidemiol 50: 275–281. Pmid:9120526
24. Xu WH, Xiang YB, Ruan ZX, Zheng W, Cheng JR, Dai Q et al. (2004) Menstrual and reproductive factors and endometrial cancer risk: Results from a population-based case-control study in urban Shanghai. Int J Cancer 108: 613–619. Pmid:14696129
25. Grattarola R (1964) The premenstrual endometrial pattern of women with breast cancer. A study of progestational activity. Cancer 17: 1119–1122. pmid:14211783
26. Iwamoto J, Takeda T. Stress fractures in athletes: review of 196 cases. Journal of Orthopaedic Science. 2003 May 1;8(3):273–8.
27. Fraser IS, Critchley HO, Broder M, Munro MG. The FIGO recommendations on terminologies and definitions for normal and abnormal uterine bleeding. Semin Reprod Med. 2011 Sep;29(5):383–90.