The human reproductive cycle is governed by a precise sequence of hormonal signals designed to prepare the body for potential pregnancy. Conception requires sexual intercourse to coincide with the narrow fertile window. The Luteinizing Hormone (LH) surge is a reliable marker for identifying this period of peak fertility. Understanding the relationship between the surge’s onset, its decline, and the actual release of the egg is fundamental to pinpointing the final hours when fertilization remains possible.
The Role of the Luteinizing Hormone Surge
Luteinizing Hormone (LH) is a gonadotropin produced and released by the pituitary gland, and its primary function is to act as the final trigger for ovulation. In the days leading up to ovulation, estrogen rises steadily, signaling that a dominant follicle containing a mature egg is ready for release.
Once estrogen reaches a specific threshold, it prompts the pituitary gland to release the large, rapid spike of LH, known as the LH surge. This surge is an intense but relatively short hormonal event, typically lasting between 24 and 36 hours, that initiates a chain reaction within the ovary. Ovulation predictor kits (OPKs) work by detecting this dramatic increase in LH concentration in the urine.
Ovulation occurs approximately 24 to 36 hours after the LH surge begins. This means the physical release of the egg often takes place just as the LH levels have peaked and begun to decline, or even after the surge has completely ended.
The Timeline of Ovulation and Egg Viability
The crucial distinction lies in the timing between the hormonal signal and the physical event. While the LH surge provides the advanced warning, ovulation usually happens between 8 and 20 hours after the surge has reached its peak. The egg is frequently released after the LH concentration has already dropped back to its baseline level.
The released egg, or oocyte, is only viable for successful fertilization for a very limited time. Once expelled from the ovary and traveling into the fallopian tube, the egg has a lifespan of approximately 12 to 24 hours.
Because ovulation happens after the LH surge, and the egg remains viable for up to 24 hours after ovulation, conception is possible even after the LH surge has ended. This post-surge period represents the final hours of the fertile window.
Defining the Conception Window
The period during which intercourse can result in pregnancy is defined not just by the egg’s short lifespan, but also by the longevity of sperm. Sperm can survive within the female reproductive tract for up to five days under optimal conditions, specifically when nourished by fertile cervical mucus.
This extended sperm viability defines the majority of the fertile window, allowing intercourse on the days leading up to ovulation to still result in conception. The overall fertile window encompasses the five days before ovulation and includes the day of ovulation itself, a six-day span.
The LH surge identifies the final two days of this window as the most fertile. The egg’s 12-to-24-hour post-ovulation lifespan is the reason why the chance for pregnancy continues after the surge has declined. Once that 24-hour limit passes, the fertile window for that cycle is closed.
Post-Ovulation Hormonal Confirmation
The definitive closure of the fertile window is marked by a clear shift in the body’s hormonal environment that occurs immediately after ovulation. Once the egg is released, the remnants of the follicle in the ovary transform into a temporary structure called the corpus luteum.
The corpus luteum begins to secrete high levels of the hormone progesterone, initiating the luteal phase of the cycle. This surge in progesterone confirms ovulation has successfully taken place, and its presence prepares the uterine lining for implantation.
The rising progesterone levels cause noticeable physiological changes, including a sustained elevation in the basal body temperature (BBT), which rises by approximately 0.5 to 1 degree Fahrenheit. Tracking this temperature shift can help retrospectively confirm when ovulation occurred.
Additionally, the sudden dominance of progesterone changes the consistency of cervical mucus, causing it to become thick and impenetrable, forming a barrier against further sperm entry. Progesterone testing is the most direct way to confirm a sustained hormonal shift. Once progesterone levels are confirmed to be rising, the possibility of conception is definitively over until the next menstrual cycle begins.