Most girls in 10-13 years old go through menarche – the first menstrual period. Of course, mothers prepare them with “the talk” in advance and try to make sure that their daughters are prudent when entering the child-bearing age.

As a rule, the talks are confined to the explanation of how the woman’s reproductive system works. During the menstrual cycle, an egg cell matures in the ovaries. If it is fertilized during ovulation, pregnancy occurs, if not – the next menstrual period comes. It would seem that everything is simple, but at times, the organism behaves in an odd way, which leads to some real "surprises."

1. One menstrual cycle and several egg cells

With the hormonal level rising at the end of menstruation, 7-8 follicles are formed in the ovary. Approximately on the 7th-9th day (in most cases), only one of them, the dominant one, continues its development. During ovulation, it releases a mature egg cell, which enters the uterine tube to be eventually fertilized. The remaining follicles, as a rule, undergo reverse development, but in some cases, several of them become dominant at once. That is, after ovulation, all the formed egg cells can be fertilized. This way, twins, or fraternal twins, are conceived.

2. Two ovulations in one cycle

Few people know that during one menstrual cycle, ovulation can occur simultaneously in both ovaries, although it is commonly believed that they alternate. There are cases when, following the first dominant follicle, the second one develops from the other side. In this case, the ovulations can remarkably be not simultaneous, but with a time gap from a few hours to a week. On the one hand, it makes the calendar-based contraceptive methods even less reliable, while it also makes it possible to get pregnant even if during the first ovulation there were no spermatozoa in the fallopian tube for some reason. If everything goes fine in both cases, the woman is expecting twins.

3. Identical twins

A zygote, or a fertilized egg cell, begins its journey from the uterine duct to the uterine cavity for further implantation. Simultaneously, there begins the process of division, forming different organs and systems, so that first it becomes an embryo and later a fetus. It is believed that the likelihood of having identical twins has to do with the genetic background, the age of parents or external factors, but this is not entirely so.

In a number of cases, a zygote is divided into two or more embryos, but it is yet not certain at what stage it occurs or what causes it. In other words, it is impossible to predict the birth of homozygous twins. These babies have almost identical sets of DNA, the same blood group, they have surprisingly similar appearance, and always belong to the same sex. They are conceived by one spermatozoon, and during pregnancy their development is supported by one placenta. At the same time, identical twins have different fingerprints, and whoever is born first is considered the eldest.

4. Siamese twins

We all heard about such cases, but few know the reasons behind such an anomaly during a multifetal pregnancy. In fact, they are identical twins, sharing some organs and parts of the body, because the division of a zygote began too late. It is also remarkable that their bodies can be joined ​​anywhere. Such children are very rare and are mostly girls. They got their name from a pair of twins from Thailand (formerly known as Siam), who toured together with a circus. In recent decades, there have been a several laborious and very lengthy operations to separate such children. Unfortunately, the survival rate is still very low.

5. It is women who choose

Another common misconception is that the first spermatozoon to get to an egg cell fertilizes it. In fact, this is only partially true. After ovulation, the egg cell enters the fallopian tube. It is captured by its fibrils and advances to the uterine cavity with the help of the ciliated epithelium and the peristaltic activity of the tube. The egg cell is incapable of moving on its own, but it doesn’t mean it is not actively involved in the process! First, it can be fertilized only by a spermatozoon of its species. Second, a group of scientists from the Pacific Northwest Research Institute, in Seattle, has provided data collected over several decades, which revealed that the egg can "read" the genetic information of each spermatozoon, reflected on its surface structures, and "reject" it if detects certain mutations. Thus, it "lets in" only for the healthiest one.

6. The embryo can develop from two egg cells

An embryo can begin to form, if the X-chromosome is artificially introduced from one egg into another. However, it doesn’t survive long, and the pregnancy will be interrupted. Virtually, all cells in the human body have 46 chromosomes. The only exceptions are sex cells – an egg cell and a spermatozoon have only 23 chromosomes each, so that their merging nuclei produce a zygote with 46 chromosomes. The egg cell is always the carrier of the X-chromosomes. And the future child’s sex is determined by the fertilizing spermatozoon. As you know, it can either be a carrier of the X-chromosome (for a future girl), or the Y-chromosome (for a future boy).

There is a natural question: What will happen if you form a zygote with a set of XX from two eggs? In case of "normal" fertilization by a spermatozoon (the carrier of the corresponding genetic material), the future child would also be female. However, in experiments of creating a zygote by artificially implanting the X-chromosome from a different egg cell, the pregnancy was interrupted at an early stage because mother’s body didn’t form the placenta. These experiments demonstrated that male genetic information is necessary for a healthy pregnancy, regardless of whether it is encoded in the X- or Y-chromosome. In addition, if mother’s genes provide for a normal development of the future baby, the father’s are required for forming the placenta.

One more interesting fact: The egg is seeking procreation, but without harm to itself. A spermatozoon contributes to the development of the placenta, which takes up most of the uterus and provides the fetus with vital substances, often to the detriment of the mother’s body. It also “needs” the bearer of its genetic material to survive.

Despite the leap in development of the modern science, even experts get stumped by some capabilities of our organism. However, it is only due to science that we are gradually disproving misconceptions that people believed for decades or even centuries.

This article is solely for informational purposes. Do not self-diagnose or self-medicate, and in all cases consult a certified healthcare professional before using any information presented in the article. The editorial board does not guarantee any results and does not bear any responsibility for any harm that may result from using the information provided in the article.