Online fertility calculators

By (embryologist) and (fertility counselor).
Last Update: 11/06/2014

At the moment of planning a pregnancy the knowledge of the ovarian cycle and fecundation is extremely useful to understand how the body works and to increase the possibilities of achieving a natural pregnancy.

Even though, there’s a lot of information available, it seems that the knowledge of how the female body functions for reproduction is not very extended, according to a study about conception and pregnancy carried out a few years ago by the company Clear Blue. According to this study half of the women asked were wrong about which moment was best to get pregnant, and a fourth of them ignored which were their fertile days.

Increasing the probabilities of pregnancy

The menstrual cycle comprehends time ever since the beginning of the period, whose first bleeding day is called DAY 1, until the day when the next period begins, which will vary between 26 or 30 days depending on the woman. The ovulation, when the eggs leaves the ovary, usually happens between 12 or 16 days after. If the menstrual cycle takes 28 days, it will happen around the day 14, if the cycle is longer, it will take more time for it to happen.

In order for pregnancy to take place, the egg needs to be fertilised in the Fallopian tubes. The egg leaves the ovary and can wait up to 48 hours until it’s too late for it to be fertilised because it’s too mature. Sperms last up to 72 hours in the female tract waiting for the fertilisation to take place. This means that the fertile period comprehends the three days before ovulation and up to three days after ovulation.

How the online fertility calculators work

Calculators make counting the fertile period of women easier, so that women can have more sexual activity during these days to increase their possibilities of getting pregnant.

In order to calculate the period it’s necessary to introduce the day when the woman started bleeding for the last time, which means the period's day 1, and the duration of the cycle, which are the days from one period to the next one.

With this data, online calculators make an estimation of the fertile days of women. However, this calculation is more accurate when women have a regular period between 21 and 35 days.

Monthly online calendar

Online calculators' effectiveness

The results are approximate and illustrative. They are done based on a calculation method called Knaus-Ogino method, a planning tool rated as being 75% reliable according to the World Health Organisation (WHO).

However, according to a recent study published by British Medical Jounal, this error margin is even higher. After carrying out an exhaustive study of menstrual cycles and ovulation days in more than 200 women they drew the conclusion that the reliability of the method is only 30%.

Fertility calculators can be a useful tool when you’re beginning to plan pregnancy, but in no way are they a substitute for the visits to the gynaecologist specialised in reproduction if pregnancy is not achieved after one year of continuous sexual intercourse in couples younger than 35, or six months of trying without success in older couples.

We make a great effort to provide you with the highest quality information.

🙏 Please share this article if you liked it. 💜💜 You help us continue!


 Laura Garrido
Laura Garrido
B.Sc., M.Sc.
Bachelor's Degree in Biotechnology from the Pablo de Olavide University (UPO) of Seville, Spain. Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI). Experience at IVF, andrology, and general analysis laboratories. Embryologist specialized in Assisted Reproduction. More information about Laura Garrido
Adapted into english by:
 Sandra Fernández
Sandra Fernández
B.A., M.A.
Fertility Counselor
Bachelor of Arts in Translation and Interpreting (English, Spanish, Catalan, German) from the University of Valencia (UV) and Heriot-Watt University, Riccarton Campus (Edinburgh, UK). Postgraduate Course in Legal Translation from the University of Valencia. Specialist in Medical Translation, with several years of experience in the field of Assisted Reproduction. More information about Sandra Fernández

Find the latest news on assisted reproduction in our channels.