The fertile window is the period in the menstrual cycle when conception is possible, encompassing the days leading up to and including ovulation. This window closes immediately after the egg is released, as the egg only remains viable for about 12 to 24 hours. Confirming the end of this fertile phase marks the beginning of the luteal phase, regulated by the hormone progesterone. Identifying this shift is fundamental for both natural family planning and optimizing the timing of medical fertility interventions. The physiological changes that occur post-ovulation are distinct and measurable, providing reliable indicators that the opportunity for conception has passed for that cycle.
The Definitive Basal Body Temperature Shift
A sustained rise in Basal Body Temperature (BBT) is a reliable at-home sign that ovulation has concluded. This increase results from the corpus luteum producing progesterone, which has a thermogenic effect that elevates the body’s resting temperature.
This thermal shift typically ranges from 0.4 to 1.0 degrees Fahrenheit (0.22 to 0.56 degrees Celsius) above the pre-ovulatory range. Tracking requires a specific BBT thermometer used orally, immediately upon waking. The temperature must remain elevated for a minimum of three consecutive days to confirm the shift.
The day of the temperature rise is considered the day ovulation occurred, retrospectively closing the fertile window. This sustained high temperature confirms the body has entered the post-ovulatory, or luteal, phase. If pregnancy does not occur, this temperature will drop back down just before the next menstrual period.
Changes in Cervical Mucus and Position
Hormonal changes following ovulation cause alterations in the physical characteristics of the cervix and cervical mucus. During the fertile window, high estrogen levels create mucus that is clear, abundant, slippery, and stretchy, often compared to raw egg whites.
Once ovulation is complete, the sharp increase in progesterone causes the mucus to change consistency. It quickly becomes scant, sticky, cloudy, or may even appear absent, creating a barrier hostile to sperm. This physical change indicates the body has transitioned into the infertile phase.
The cervix also changes position and texture due to the hormonal shift. During the most fertile time, the cervix is high, soft, and its opening (the os) is slightly open.
After ovulation, the cervix drops lower and becomes much firmer, similar to the tip of the nose, while the os closes tightly. This firm, low, and closed state confirms the uterus is secured for the luteal phase.
Testing for the Post-Ovulatory Progesterone Rise
Testing for the rise in progesterone is the most direct method for chemical confirmation that ovulation has ended. Unlike Luteinizing Hormone (LH) tests, progesterone tests confirm the event has successfully concluded, which is important because an LH surge does not always guarantee an egg was released.
At-home testing measures Pregnanediol Glucuronide (PdG), a progesterone metabolite excreted in the urine. PdG levels rise significantly 2 to 3 days following ovulation, and sustained positive readings confirm adequate progesterone production.
A healthcare provider can order a serum progesterone blood test, typically scheduled seven days after suspected ovulation to capture the hormone at its peak concentration. A level above 5 to 10 nanograms per milliliter (ng/mL) confirms that the corpus luteum is functioning and that ovulation has occurred.