You need two things to get pregnant: one egg and one sperm. That’s it.
NaturalCycles is the number one app when it comes to finding your correct ovulation day. This is the day when your egg is released from your ovary and the only day when you actually can get pregnant. We carefully monitor your temperature fluctuations and hormone concentrations throughout your menstrual cycle in order to determine when ovulation occurs. Since an egg survives between 12 and 24 hours, we can rapidly give you green days for the rest of your cycle. But how do we handle sperm survival?
How long does sperm survive?
In order to work as an effective contraceptive, Natural Cycles needs also to take into account that sperm survives a few days inside the female body.
It is well known that sperm can survive up to a maximum of 5-6 days in the female body. You may hear different things from different sources, but there are only 6 days per cycle where an act of unprotected intercourse may lead to a pregnancy check out the original research paper on sperm survival here.
Your fertile days (fertile window) are only six days long if the sperm encounters the right environment in the vagina, uterus, and fallopian tubes. In order for the sperm to survive longer than a few hours, you need to have a fertile cervical fluid. Without that, sperm will rapidly die and the fertile window will be reduced to a few hours during ovulation day. The fertile cervical fluid will help the sperm to travel from the cervix to the uterus and finally to the fallopian tube. Once sperm reaches the fallopian tube, it can live there for up to 5 days waiting to fertilize an egg. Analyzing your cervical fluid is a good fertility indicator and can be taken ‘Note’ of in the app. However, Natural Cycles does not take it into account when calculating your status for the day as it is not objective, takes practice to identify and can be influenced by factors such as sexual arousal secretion.
Regardless of whether your cervical fluid is fertile or not, Natural Cycles always assumes that your fertile window is 6 days long – the maximum length. This means that NaturalCycles will never give you less than 6 red days per cycle. Though it has to be said that generally speaking you can expect about 10 red days per cycle due to irregularities etc.
How does NaturalCycles deal with irregularities in your cycles?
OK, so we take sperm survival into account. Another comment we hear a lot from potential users that want to be sure about the safety of our method is the following. “The body temperature rises after ovulation, which marks the end of the fertile window. This means that until we detect ovulation, we don’t know whether we are inside or outside the fertile window”. This is only partially true. NaturalCycles predicts at the beginning of each new cycle the expected ovulation day. If you are in your first cycle, NaturalCycles doesn’t know yet when you ovulate. Unlike other apps, we do not make the general assumption that your ovulation day equals to your average cycle length minus 13 days (which is the average luteal phase of women). In your first cycle, you can therefore mostly expect red days until ovulation. If you record more cycles with NaturalCycles, we know your average ovulation day as well as how much it varies. NaturalCycles then will make sure to cover your fertile window with red days taking into account the irregularities. In fact, we make sure that we cover 99.9% of all possible scenarios. It means that there is only a 0.1% chance that a green day should actually have been a red day. That’s why we can say that we are 99.9% safe. If you multiply this by the actual probability of getting pregnant (which is 25% on your most fertile day), you can rest assure that you can really rely on NaturalCycles. Depending on how irregular your cycles and your data are, you can expect more or less red days. But you will always be on the safe side during green days!