Ovulation is the moment when an ovary releases a mature egg into the fallopian tube, where it can be fertilized by sperm. It typically happens once per menstrual cycle, roughly midway through, and the released egg survives only 12 to 24 hours. That narrow window, combined with sperm’s ability to live up to five days in the reproductive tract, creates a fertile period of about six days per cycle.
How Ovulation Is Triggered
Ovulation is the end result of a hormonal chain reaction that starts in the first half of your cycle, called the follicular phase. During this time, several follicles (small fluid-filled sacs, each containing an immature egg) begin developing on the ovaries. Usually one follicle outpaces the rest and becomes dominant, producing rising levels of estrogen as it grows.
When estrogen reaches a critical threshold and stays elevated for roughly 50 hours, it triggers a dramatic surge of luteinizing hormone (LH) from the pituitary gland. This LH surge is the direct trigger for ovulation. Once it begins, the egg is released about 34 to 36 hours later. More precisely, ovulation occurs roughly 10 to 12 hours after LH hits its peak. The surge also causes the egg to complete its final stage of cell division, making it ready for fertilization.
When Ovulation Happens in Your Cycle
Clinical guidelines typically place ovulation around day 14 of a 28-day cycle, but real-world timing varies far more than most people realize. A large prospective study published in the BMJ found that ovulation occurred as early as day 8 and as late as day 60. Even among women with regular 28-day cycles, the fertile window most commonly fell between days 8 and 15, not neatly centered on day 14.
Only about 30% of women have a fertile window that falls entirely within the textbook range of days 10 to 17. Women with shorter cycles (27 days or less) tend to ovulate earlier. About one third of those women had already entered their fertile window by the end of the first week. Women with longer cycles ovulate later, and a small percentage (4 to 6%) were still potentially fertile into the fifth week of their cycle. The takeaway: if you’re relying on calendar math alone, the margin of error is significant.
The Fertile Window
Your actual window for conception is wider than the egg’s short lifespan might suggest. Sperm can survive in the reproductive tract for up to five days, so intercourse in the days leading up to ovulation can still result in pregnancy. The fertile window is generally considered to be the five days before ovulation plus the day of ovulation itself. The highest probability of conception comes from the two days before ovulation and the day it occurs.
What Happens After the Egg Is Released
Once the egg leaves the ovary, the now-empty follicle transforms into a temporary structure called the corpus luteum. This structure produces progesterone, the hormone that dominates the second half of your cycle (the luteal phase). Progesterone thickens and prepares the uterine lining so a fertilized egg can implant. It also raises your body temperature slightly and stops the uterine lining from continuing to grow.
The corpus luteum has a built-in lifespan of about 11 to 17 days, averaging around 14. If a fertilized egg implants and begins producing pregnancy hormone (hCG), the corpus luteum is “rescued” and continues producing progesterone to sustain the early pregnancy. If no implantation occurs, the corpus luteum breaks down, progesterone drops, and the uterine lining sheds as your period.
Physical Signs of Ovulation
Some people notice clear physical changes around ovulation, while others feel nothing at all. The most reliable external sign is a shift in cervical mucus. In the days leading up to ovulation, discharge becomes wet, stretchy, and slippery, resembling raw egg whites. This consistency helps sperm travel more efficiently toward the egg. After ovulation, mucus typically becomes thicker and less noticeable.
About one in five women experience ovulation pain, sometimes called mittelschmerz. This is a dull ache or sharp twinge on one side of the lower abdomen, on the side of whichever ovary is releasing an egg that month. It typically lasts a few minutes to a few hours, though it can occasionally persist for a day or two. The pain likely comes from the follicle stretching the ovary’s surface before it ruptures, or from fluid released during the rupture irritating nearby tissue. Some women also notice light spotting around ovulation.
How to Track Ovulation
Home ovulation predictor kits (OPKs) detect the LH surge in your urine. Since the surge begins roughly 34 to 36 hours before ovulation, a positive result tells you the egg is coming soon. A 2024 study comparing five popular brands found that all had overall accuracy above 91% when compared to blood LH measurements. Sensitivity (the ability to correctly detect a true surge) varied more, ranging from about 39% to 77% depending on the brand. Testing once daily can sometimes miss a short surge, so many people test twice a day as they approach their expected ovulation date.
Basal body temperature (BBT) tracking works differently. Progesterone released after ovulation raises your resting temperature by about 0.5 to 1 degree Fahrenheit. You can detect this shift by taking your temperature first thing every morning before getting out of bed. The limitation is that the temperature rise confirms ovulation has already happened, so it’s more useful for understanding your pattern over several months than for predicting ovulation in real time. Combining BBT with mucus tracking and OPKs gives the most complete picture.
What Can Prevent or Delay Ovulation
When ovulation doesn’t occur during a cycle, it’s called anovulation. Polycystic ovary syndrome (PCOS) is one of the most common causes. In PCOS, hormonal imbalances prevent follicles from maturing fully, so no egg is released. Higher body weight is closely linked to PCOS, present in 54 to 68% of cases, and insulin resistance plays a central role in disrupting the hormonal signals needed for ovulation.
Thyroid disorders, particularly an underactive thyroid, can also interfere. Hypothyroidism raises levels of a hormone called prolactin, which disrupts the balance of reproductive hormones and can inhibit ovulation entirely. The connection runs deep enough that hypothyroidism can even produce ovary changes that mimic PCOS on an ultrasound.
Beyond medical conditions, high levels of physical or psychological stress, significant weight loss, excessive exercise, and breastfeeding can all suppress the hormonal signals that trigger ovulation. Hormonal birth control works by deliberately preventing ovulation through synthetic hormones that override the natural LH surge. Once you stop these methods, ovulation typically resumes within a few cycles, though the timeline varies.