• How many women have a cycle of normal length?
  • What is the individual fluctuation range of cycle lengths within one year?
  • How long can the follicle phase last?
  • How do most couples worldwide practice contraception 
  • How does illiteracy restrict knowledge about their own bodies?
  • How do rates of fertility differ in countries all over the world and how has it changed over the last few decades? 

The science of epidemiology deals with all these questions.

Statistics on the female body

There is no "normal" cycle – every woman has an individual cycle. It shows natural fluctuations and changes over the course of a lifetime. That is, the rhythm of fertile and infertile days can vary from woman to woman and from cycle to cycle.

Generalizing statements about when a woman is fertile are therefore problematic. This is shown, for example, by a study by the University of Düsseldorf analysing data of 210 healthy women with a total of 10,000 cycles from the German cycle database. 

It was found that the "classic 28-day cycle" is far rarer than it was believed. In women who have a natural cycle, i.e. who do not use hormonal contraceptive methods, only about 13% of the cycles last exactly 28 days.

Healthy adolescents are even less likely than adult women to have a cycle length of 28 days. In fact, the most common cycle length in women between the ages of 19 and 45 is 27 days.

All cycle lengths between 23 and 35 days are considered "normal". Longer cycles  tend to be more common than shorter cycles. 

Looking at the individual woman and her cycle lengths within a year, it's obvious that her cycles are not always the same length, to the contrary:
In 60% of all women, the cycle length fluctuates by more than one week within a year. Of these 60%, almost one in two experiences fluctuations of more than two weeks over the course of a year. In almost 30% of all healthy women, for example, the cycle length varies between 25 and 39 days or between 29 and 43 days. 

Variations in cycle length are therefore part of healthy women's cycles and do not say anything about whether there is a cycle disorder to start with. Therefore, it is also not advisable to define the fertile and infertile days on the basis of a rigid 28-day cycle scheme. They can only be determined assessing the individual cycle course.

The time from egg maturation to ovulation is not a constant. It lasts varying in length from woman to woman and varies from cycle to cycle in a single woman. If you look at an entire cycle, it can be divided into two phases: the first phase lasts from the first day of bleeding to ovulation. It is called the egg maturation phase (follicle phase). The 2nd phase lasts from ovulation to the day before the next bleeding.

The so-called "egg maturation phase", i.e. the time until ovulation, is primarily responsible for the length and variation of a cycle. It can last from one week to many weeks.

The duration of the 2nd cycle phase is fairly constant (12-16 days). In half of all cycles, ovulation occurs only after the 14th day of the cycle. In one in five cycles, ovulation only takes place on the 20th day or later. Young women in particular are more likely to have longer cycles and thus later ovulations: among 20- to 25-year-olds, cycles with ovulation taking place on the 20th day or later are almost one in four. This means that ovulation often only takes place in the third, fourth or fifth week of the cycle. In addition, younger women have much broader variations in their cycle length than older women. 

Fertility // global

In South Korean fewer than ever children have been born since the country started collecting data nearly 50 years ago. Statistically, a South Korean woman has only 0.98 children, down from 1.05 in 2017. That's according to government data for 2018. As a result, the fertility rate in South Korea is at a record low. In 1970, when South Korea first statistically recorded the birth rate, it was 4.53. Since then, it has fallen dramatically: in 2010, the birth rate per woman was 1.23, and in 2016 it was 1.17.

Japan recently suffered its latest demographic blow when a report was published showing that in 2019 the country had the lowest birth rate since records began in 1899. The report, published by the Department of Health, Labour and Social Affairs, found that the number of births in 2019 was just 864,000.

And falling rates are not only to be found in Asia. Most G7 countries see similar demographic trends. According to a Pew Research report, Italy recorded the lowest total number of newborns in 2018, while the US recorded the lowest number of newborns in 32 years due to a steady decline over many years. The situation in developing countries is different. The fertility rate in Africa in 2019 was about 4.5 births per woman, with the birth rate in Niger at 6.62 and in Kenya at 3.14. Overall, the birth rate in Africa has fallen from 5.1 births per woman to 4.5 since the beginning of the millennium.

In addition, almost 100 million women worldwide become unintentionally pregnant every year. A study in Lancet Global Health (2018) puts the share of unwanted pregnancies in all pregnancies at 44%. More than half of unwanted pregnancies end prematurely. However, unwanted pregnancies are not only of medical interest, as illegal abortions often endanger women's health. Children born after an unwanted pregnancy also suffer disadvantages. A US study found that mothers are less likely to breastfeed their unwanted children.

Contraception // global

In the 1960s, the population of developing countries grew rapidly. Back then for the first time the fast global population growth was perceived and discussed internationally. The term "population explosion" also dates back to this period. Developing countries had little experience with family planning at the time. Having children was a natural law and most women became pregnant as often as it would happen, so on average women had six to eight children.

This high level of fertility was among other reasons a consequences of the very high infant mortality rates until middle of the 20th century. Improvements in medical care in the 1960s led to a rapidly declining infant mortality rate in developing countries. The result was strong population growth. Family planning was an obvious answer. With the help of modern contraceptives, it became possible to determine the number of children, the time of birth and the distances between the individual births. However, while in developed countries well above two-thirds of women between the ages of 15 and 49 use modern methods to prevent pregnancies, in Africa their share is only 23%.

However, there are large differences between North Africa (44%) and sub-Saharan Africa (15%). The governments of sub-Saharan Africa were the last to create the legal bases for promoting family planning and reducing fertility rates. Population growth is therefore still very high in this region. This relates to the number of children being considered a status symbol in sub-Saharan Africa and many children being also regarded as pensions. In addition to the traditional values and patriarchal character of many communities there is a high rate of polygamous marriages. 

When contraceptives are used, women in Africa are more likely to prefer the pill or an injectable contraceptive. Syringes are that popular because they can be used without the husband's knowledge.

A regional comparison also shows that in South Africa, for example, 53% of all couples use modern methods of family planning, compared to only 9% in West Africa. In North Africa, the situation is different again. Egypt and Tunisia have been able to increase the use of contraceptives enormously through national support. In this region, pill and coil are preferred

Illiteracy // global

According to UNESCO's 2016 World Education Report, some 750 million people worldwide are illiterate. About two-thirds of them are women. About 500 million are spread over only ten countries, half of them in India alone. The lowest literacy rate worldwide in 2016 was in the sub-Saharan Africa region. Only 65% of all people in this region can read and write. Additionally, these were the regions with the most pronounced differences in literacy between men and women (alongside North Africa, Western and South Asia).

Globally, however, there has been a greater increase in female literacy than men since the 1960s. This was particularly evident among women aged 15 to 24, almost 91% of whom had at least basic literacy skills.


  • Zyklusdatenbank, University of Düsseldorf, 2008
  • UN, Department of Economic and Social Affairs, Population Division, 2010 -2015
  • CIA Factbook,  2016
  • Demographic and Health Surveys Program, United States Agency for International Development (USAID)
  • Lancet Global Health (2018; doi: 10.1016/S2214-109X(18)30029-9)
  • Am J Public Health 1997 October; 87(10): 1709–1711
  • UNESCO, world science report, 2016