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Blood group

What is a blood group and how is it determined?

A person’s blood group is determined by either presence or absence of special compounds, antigens. They are conventionally designated by Latin letters (A and B). 4 blood groups were identified according to the antigens’ presence or absence. Scientists have recently proven that in fact there are many more blood groups. However, the so called AB0 system is still used for blood transfusion.

blood groups are defined as follows


According to the system, the blood groups are defined as follows:

  • I (0) is characterized by the absence of antigens A and B;
  • II (А) is characterized by the presence of antigen A;
  • III (В) is characterized by the presence of antigen B;
  • IV (AВ) is characterized by the presence of antigens A and B.

To determine a child’s blood group by that of its parents, genetic methods are used, therefore determination is not difficult. It is merely required to apply in practice Mendel's laws learned in school biology lessons. According to these laws, blood groups are inherited as follows.

  • If the parents have the 1st group, their children will have the same, since none of the parents has antigens I (0) in their blood.
  • If one of the spouses has the 1st group, and the second spouse has the 2nd group, the children may also inherit the 2nd group, since one of the parents does not have antigens in the blood, while the second parent will transmit antigen A responsible for the 2nd blood group.
  • A similar situation is observed when one of the parents has the 1st group, and the second one - the 3rd group. In such a case, the child may have either the 1st or 3rd group.
  • If one of the parents has the 3rd group and the second one – the 2nd group, the child may have either blood group with equal probability (25%).
  • The 4th blood group is rather rare, since for a child to have such blood, 2 antigens must be present simultaneously.

You may use this Table to determine the blood group of a child based on that of its parents. The red color designates the parents’ blood group, and the blue color at the intersection designates the blood group of the future child.

How is the rhesus factor inherited?

The term rhesus factor means the protein present in the blood of 85% of all people. Those people whose blood contains the protein are Rh-positive. Otherwise the blood is called Rh-negative. To determine such a parameter as rhesus factor of a child by the parents’ blood group, genetic laws are also used. A pair of genes usually designated as DD, Dd, dd is sufficient for the investigation. The capital letters mean gene dominance, i.e., are used for designating those who have the rhesus protein in their blood. If the parents have heterozygous rhesus (Dd), then in 75% cases, the children will also be Rh-positive, and only in 25% cases - Rh-negative.

Heterozygosity occurs by a child as a result of the so-called conflict of the mother’s rhesus-negative factor and may be transmitted to many generations. However, in most cases, this does not occur, because pregnancy is very unlikely to occur in such conditions, and if it occurs, it ends with an early abortion.

The rhesus factor is determined as follows:

  • A child will definitely have negative rhesus, if both of the parents also have negative rhesus
  • In all other cases, the rhesus factor may be either negative or positive