The Ogino-Knaus method
The Ogino-Knaus method for calculating a woman’s fertile period was born as a contraceptive system, but it can also be used to identify the fertile days of the cycle in a probabilistic way , i.e. those close to the moment of ovulation .
The method, drawn from the independent observations of a Japanese doctor and an Austrian gynecologist in the 1920s, is based on statistical calculations and therefore cannot determine the exact day of ovulation , but only the period of greatest probability .
For this reason, the Ogino-Knaus method is not recommended as a contraceptive system . If, on the other hand, you use it to try to get pregnant… obviously you don’t run any risk!
The calculation
The Ogino-Knaus method is based on a relatively simple calculation: knowing the length of one’s cycle , 14 days are subtracted from the presumed date of the next menstrual flow and the day obtained is the central one of the probable ovulation period. The complete period, according to Ogino-Knaus, goes from two days before to two days after the central one .
The calculation uses the estimated day of the next period , which therefore can only be known by women who have a regular cycle . If your cycle length is highly variable , counting your probable ovulation time becomes unreliable .
The periods obtainable with the calculator
When the length of the cycle is regular, if the date of the last menstrual period is known, the (presumed) start date of the next flow is also known. By applying the Ogino-Knaus formula, the cycle can then be divided into different parts , with different levels of fertility .
Infertile days
They are those far from the period of ovulation, during which sexual intercourse should not initiate a pregnancy. But be careful : nothing is guaranteed and mother nature could reserve some surprises. In case of anticipated or delayed ovulation or if the cycle is irregular , the calculation could in fact give incorrect results.
The fertile days
They are those in which sexual intercourse could lead to pregnancy. These days do not necessarily correspond to when ovulation will occur. In fact, sperm can survive 3 to 5 days in a woman’s body . Consequently, even if ovulation occurs a few days after intercourse, fertilization could still occur.
Ovulation is not after 14 days
It is a widespread habit to think that, according to the Ogino-Knaus method, ovulation occurs 14 days after the start of the cycle . This is true, always statistically , only if:
- the menstrual cycle lasts 28 days ;
- the luteal phase (ie the period from ovulation to the beginning of the next cycle) lasts 14 days .
However, many women have a shorter or longer menstrual cycle than the average and, in part, the same goes for the duration of the luteal phase, even if this occurs more rarely and only in selected cases.
If the cycle is 28 days, the Ogino-Knaus method will effectively give the 14th from menstruation as the central day of the fertile period, but if the cycle is 29, the day will become the 15th and so on.
In fact, the calculation must be done in reverse : 14 days are subtracted from the (presumed) start of the next cycle and not from the start of the previous one.
Other methods to detect ovulation
For those trying to get pregnant, the Ogino-Knaus method can be useful as a first approach, but it is not very accurate .
Other systems for recognizing ovulation are more reliable and also help to get to know your body and your menstrual cycle better :
- the basal temperature method
- the cervical mucus method (Billings)
- the method of the position of the uterine cervix
- ovulation test sticks
Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.
Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.