The fertile window

 

The woman’s body is really a remarkable thing. Cycle after cycle, the uterus does its job and releases another egg, expecting it to be fertilized. Today, since we have such great knowledge through medical research on what’s going on in the uterus, we can use it to our advantage to either prevent or plan a pregnancy. The time frame when a woman is fertile only occurs once per cycle and is called the fertile window.

The fertile window includes the 5 days prior to ovulation and the day of ovulation.

Fertile Window - NaturalCyclesThe 5 days period prior to ovulation is determined from the longest time sperm can survive in the uteral environment. Note, however, that most sperm have a lifetime significantly less than 5 days – more like 2 or 3 days, but to completely exclude a possibility of pregnancy one must take the longest living sperm into account. For sperm to survive any significant time at all, the uterus must contain the fertile type of cervical mucus, which helps the sperm to live longer as well as to be able to travel up the uterus and fallopian tubes. Without the presence of fertile cervical mucus sperm typically only survive about 4 hours.

The fertile cervical mucus is triggered by a rise in estrogen prior to ovulation. The amount of fertile cervical mucus does not only vary from woman to woman, but also on the age of the woman. The older you get, the fewer days you produce cervical mucus and hence the narrower your fertile window becomes.

Once released through ovulation, the egg can maximally survive up to 36h, but typically only 12-24h. Studies have shown that the quality of the egg deteriorates very quickly; causing the probability of conception to decrease rapidly every hour once the egg has been released. For optimal chances of conception, sperm should thus already be present in the fallopian tube once the egg is released. Therefore, the most fertile day of the woman’s cycle is rather the day prior to ovulation than the day of ovulation itself.

To prevent pregnancy through detecting ovulation and predicting the fertile window, one must assume the longest living sperm and egg. This sums up to 6 days in the cycle. What’s more tricky is to accurately calculate the uncertainty of the ovulation day for a woman. That is, when do we think she will ovulate and what’s the earliest possibility of ovulation to occur. Luckily this is what NaturalCycles‘ algorithm does for you. When you start measuring your temperature, NaturalCycles will be very cautious as it does not know around what time you usually ovulate. With more and more data, NaturalCycles is able to better estimate your ovulation day, the variation of your ovulation day and the uncertainty on the estimated variation of your ovulation day. All this is required for a full-proof birth control method using natural family planning. So don’t chart by hand to prevent getting pregnant ladies – it is doomed to end up in an accident. Use the mathematical tools provided for you and you’ll save both time and hassle.