Ovulation occurs when an egg is released from the ovary. It proceeds down the fallopian tube to the uterus in hopes of being fertilized. Pregnancy results if the egg is successfully fertilized by a sperm and implants in the uterine lining. Fertility is highest at the time of ovulation.
The following image shows the likelihood of intercourse resulting in conception depending upon when it occurs relative to conception. For example, "Day -5" corresponds to five days prior to ovulation. This is loosely derived from actual clinical data sets which test extensively so that ovulation is known precisely. In practice, the likelihood of conception is actually slightly wider by a day or two due to possible uncertainty of the exact date of ovulation. Charting multiple symptoms can help reduce this uncertainty if the symptoms all agree on the same day for ovulation.
Fertility During Ovulation
Fertility peaks at ovulation. Intercourse that coincides with ovulation offers the highest probability of conception. The egg only is only available for fertilization for less than 24-hours, so this peak window of opportunity is relatively small.
Fertility Before Ovulation
Prior to ovulation, the egg is not yet ready for fertilization. Fertility is still high though because sperm can survive for up to 6 days. This is possible because cervical mucus undergoes changes right around ovulation to help encourage sperm survival. Sperm introduced during intercourse three or more days prior to ovulation may still result in conception if they survive long enough.
Increasing estrogen levels are responsible for the changes in female cervical mucus. These changes create a friendly environment for sperm and allow them to survive much longer than otherwise possible. This widens the fertile window and makes it possible for intercourse days prior to ovulation to still result in conception. This is important to consider when trying to achieve or avoid pregnancy.
Cervical mucus is one of the best indicators of approaching fertility. Charting it is particularly valuable for couples trying to conceive. More information about charting cervical mucus can be found here.
Fertility After Ovulation
After ovulation, fertility drops quickly. Once the egg is no longer available for fertilization, the opportunity for conception passes. Couples trying to avoid pregnancy can use this fact to resume unprotected intercourse with minimal risk of pregnancy.
Guidance for Considering Ovulation Uncertainty
Fertility charting using fertility awareness techniques makes it possible to detect ovulation. Women can record symptoms and chart them to determine a likely ovulation date. In practice this date may differ from the actual day of ovulation by a day or so. When charting multiple symptoms, it is not unusual for individual symptoms to suggest different ovulation dates. These dates are usually very closely spaced though (often within a day of each other). Sometimes they will even independently suggest the same day of ovulation. This raises the confidence in the chosen ovulation date.
This variability in detecting ovulation slightly widens the fertile window. This helps compensate for possible variations in the actual ovulation date. This should be kept in mind when considering pregnancy odds. The figure above does not consider this, although fertility charting software often will.
This uncertainty can be caused by variability in recorded symptoms among other factors. The idea that ovulation occurs on a specific date also fails to take into consideration the time of ovulation. The less than 24-hour window where the egg can be fertilized may not necessarily correspond to exactly one day. It could span part of one day and into the next. This collectively explains the need for additional variation beyond the more ideal assessment of fertility depicted above.
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