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Calculating your
fertile days

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Some things can help to be taken into account when planning a pregnancy. In addition to the general health aspects such as diet and lifestyle, it is helpful to understand when ovulation occurs and which days of a woman's cycle are the most fertile, in order to be able to adjust one's behaviour accordingly.

When does ovulation occur?

The menstrual cycle is composed of an egg maturation phase and a luteal hormone phase. Both are controlled by the brain with the follicle-stimulating hormone FSH and the luteinizing hormone LH. Thus, with an optimal cycle of 28 - 30 days, ovulation can be assumed to occur between the 12th and 14th day.


Ovulation is triggered about 24 hours before the egg is relased by the release of LH from the pituitary gland. After that, the egg is only capable of fertilization for 12 to 24 hours and then perishes.

When are women fertile?

Since sperm can survive in the woman's body for up to 7 days, it is important to know that the fertile days are longer than the fertilization ability of the egg (12 - 24 hours). Thus, the most optimal time for fertilization occurs in a time window of about 3 days before ovulation and about 12 hours after.

Cycles of 25 - 35 days can be considered normal. If there are repeated deviations from this, cycle monitoring should be performed.

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Calendar method (Knaus-Ogino method)

Logging the cycle length over a few months (Knaus-Ogino method) can provide some information about regularity and ovulation timing. This can also be done electronically. Since the cycles usually show deviations, the length of up to 12 cycles is recorded. From the shortest cycle of the series, 18 days are subtracted and the first fertile day is obtained. From the longest cycle of the measurement series, 11 days are subtracted and the last fertile day is obtained. Thus, a conclusion about the fertile days can be drawn retrospectively from the evaluated cycles.

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Basal temperature measurement

The release of luteal hormone (progesterone) after ovulation can cause an increase in core body temperature (basal body temperature) of 0.3 to 0.5 degrees Celsius. By measuring body temperature daily in the armpit immediately after waking from cycle day 8, the increase will be noted sometime after ovulation. An increase can be assumed if there is a plus difference of at least 0.2 degrees Celsius on 3 consecutive days compared to the last 6 days before the increase. If the temperature rise has occurred, the fertile window is over. Thus, an evaluation can only be carried out in retrospect.

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Analyze cervical mucus

Due to hormonal changes during egg maturation, the production of mucus from the uterus also changes. This cervical mucus can be seen and felt on or in the vagina. At the beginning of the cycle, there is hardly any discharge. However, as the cycle progresses, it increases and clears up. Just before ovulation, it feels increasingly runny and stringy. This is a sign of estrogen production by the maturing egg and correlates with impending ovulation.

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Hormone analysis

Another method to narrow down the time of ovulation or the fertile days is to directly measure the hormones that trigger ovulation. The release of LH in the urine is used for this purpose. It is assumed that ovulation occurs about 24 to 36 hours after the maximum LH release.

The technical procedure is comparable to a pregnancy test. The measurement should always be taken at the same time of day and starts around the 10th day of the cycle.

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What to do if pregnancy does not occur?

Infertility is when a couple has not achieved pregnancy for over a year despite regular sexual intercourse. In this case, our reproductive medicine experts in our fertility clinics in Vienna, Wels and Klagenfurt get to the bottom of the unfulfilled desire to have a child with comprehensive diagnostics. With our own diagnostic laboratory, cryobanks and sperm bank, we can offer you the full range of fertility tests (hormones, genetics, immunology), fertility treatments and other supportive treatments.

Find out more at these TFP clinics:

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