An egg is only viable for somewhere between 12-24 hours, and once it is ovulated it must be fertilized in that time otherwise it will begin to disintegrate.
However, pinning down ovulation is not an exact science.
If a woman is tracking some combination of her LH surge, cervical mucus, basal body temperature, bloodwork for estrogen and LH, and maybe even getting ultrasounds, she can have a pretty good idea.
But several things will make the exact timing trickier to tell. Some women will ovulate the same day of an LH surge, or up to 48 hours later. Their temperature might take a few days to rise. It may vary month to month. One interesting study showed that some women ovulated BEFORE an LH peak, so I strongly recommend cross checking more than one symptom to estimate both the opening and closing of a fertile window.
We need to have sperm lined up and ready for an ovulated egg, no matter what the timing might be. Luckily, sperm can survive up to 5 days within the uterus and fallopian tubes.
This is why we talk about a “fertile window” instead of an exact time. Statistics show that timing intercourse 1-3 days prior to ovulation actually gives the best chance of conception. Since it is challenging to know ahead of time what 2 days prior to ovulation might be, we recommend that a woman considers her fertile window open as soon as certain signs line up: cervical mucus changes, LH starts to gain colour on an LH strip, a woman has a general idea of the time of month she ovulates, estrogen starts to rise, etc.
It allows time for the sperm to make the trip from the vagina, through the cervix, into the uterus, and up the fallopian tubes to wait for ovulation to occur. Many sperm will not make it to the right areas, and that is part of the reason we need many millions of sperm for their high rate of attrition.
Studies show that timing intercourse every 2-3 days through the estimated fertile window (approximately 1 week or more) is the best strategy to both ensure fresh sperm, and also allow for sperm count to replenish in between.
Increasing the frequency of insemination does not increase pregnancy outcomes.
The fertile window closes when estrogen decreases (on bloodwork), or physically you will notice cervical mucus dry up (since it is related to estrogen). Progesterone will then begin to rise, and your luteal phase begins!
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