Calculating ovulation, fertile days, and infertile days – HEALTH

Calculating ovulation, fertile days, and infertile days

How do you recognize fertile days?

First things first: without special tests or examinations, it isn’t possible. Even pregnancy computers that determine the temperature and consistency of cervical mucus can only approximate fertile days.

How does pregnancy work?

In order to become pregnant, sperm must unite with the egg in the fallopian tube. In this context, the most important date is the ovulation itself. At this point in time, the woman ovulates, which means one or more eggs are released from one of the woman´s ovaries. Here’s the problem: ovulation can’t be determined with 100% accuracy. In a 28-day cycle, for example, ovulation occurs between the 13th and the 17th day. Unfortunately, very few women have cycles of a consistent length.

Sperm can survive 3 to 5 days. The egg itself can survive about 12 to 18 hours. This means that the fertile days in a 28-day sample cycle lie between the 8th day and the 18th day.

On what day does pregnancy become possible?

After ovulating, the luteal phase starts: eggs are fertile for a mere 12 to 18 hours. Sperm, by comparison, live much longer: about 3 to 5 days. Overall, this results in about 6 fertile days (the day of ovulation and the five days before it).

Since the exact time of ovulation can’t be determined precisely, you should anticipate a few safe days if you don’t want to become pregnant.

The cycle

The cycle length is nothing more than the interval between two cycles. The first day of menstruation is the beginning of a new cycle. Unfortunately, only one moment in the cycle is easy to determine: the beginning (because of menstruation).

Normal cycle length

There is no “normal” standard cycle! Every woman has a unique cycle that may also fluctuate from month to month. Additionally, cycle length tends to vary throughout life. Very few women have a consistent cycle length. Irregular cycles are no cause for worry; they are considered normal.

Varying cycle lengths

Generalities about cycles are difficult. For one, fertile and infertile days vary between women. Additionally, cycle lengths vary for each individual woman. For these reasons, it’s better to observe bodily changes to determine fertile days than it is to use calculations. This can be done by a physician or with individual training.

Average cycle length

According to studies, the supposedly common 28-day cycle is seen much less often than expected. Only 12.8% of all cycles last 28 days. Additionally, adult women are more likely to have 28-day cycles than juveniles.

The most common cycle length in women between 20 and 45 years old is 27 days. All cycles between 23 and 35 days are considered medically normal (!). Here’s another fact: longer cycles are more common than short ones. Every 6th cycle lasts 33 days or more. Every 10th cycle lasts 24 days or less. Five percent of cycles in healthy women last more than 35 days.

Calculating fertile and infertile days

Variations in cycle length are completely normal. An individual consultation with a gynecologist makes the most sense for determining the subdivision of fertile and infertile days.


Ovulation usually takes place between the 12th and the 16th day before the next period. During this short period, it is possible to conceive a child.

Menstrual cycles and fertility

The first five days of a period are considered infertile. But this is only true without hormonal influences (the pill!) and if ovulation actually occurred in the previous cycle. From the sixth day of a cycle, fertility is at least theoretically possible.

Beware of traps: myths about fertile days

Unfortunately, there are still textbooks and instructional brochures that spread false information or greatly simplify how things work. For some people, condoms are initially used for contraception. But since they’re perceived as annoying, these people don’t use them at the start or end of a cycle because these are allegedly safe days. Sadly to say, that kind of thinking is mistaken!

The following blanket statements are false

  • Fertility is highest halfway through the standard cycle.
  • The best time for conceiving is exactly halfway between two menstrual periods.
  • Sperm can only survive for 2 to 3 days at most.
  • Pregnancy is impossible during menstruation.
  • Contraception is unnecessary until the 7th day of the cycle.
  • The standard cycle is 28 days long.
  • Ovulation always occurs in the 14th day of the cycle.
  • The days immediately after menstruation (7th to 9th days of the cycle) are still relatively safe.
  • After ovulation, which takes place no later than the 18th day of the cycle, it’s impossible to get pregnant.
  • The week before menstruation (22nd to 28th days of the cycle) is 100% safe.


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