This method is based on the theory that the blood in the human body has the ability updates. A full cycle of this process, the male is 4 years, women – 3. Of course, this is relative numbers, and it can happen much faster, especially if in a person's life there are a variety of events associated with the loss of blood, for example:
- it will transfusions;
- blood donation to blood donation;
- when a sharp and fairly heavy blood loss due to surgery, trauma, abortion.
These factors are crucial in the calculations. If you do not take them into account, the exact sex of the baby to see blood has not come out.

## Rules of calculations of the sex of the baby

This theory has a very simple calculations. To upgrade blood to know the sex of the child, you need the exact age of the parents divided by the number of years for which the blood is able to be updated. That is the age men we divide by 4 women in 3. If someone of the parents not long ago had circumstances, accompanied by great loss of blood, you should begin it from this moment and consider it the last renewal date.

According to the theory, the sex of the child will correspond to blood of the parents at the time of conception was younger. For example, if the father's blood was renovated 2 years ago, and mother 3, so it's a boy.
In cases if the blood of father and mother, was updated in very close to each other terms, the probability of conceiving twins or fraternal twins increases significantly.

## If the theory is correct

Still no one is 100% not proven, is it possible to learn to update blood the sex of a baby. A couple of respondents argue that the theory has really helped them to predict and plan the sex of the baby. But the overall statistics say that the theory is correct in only 50-60% of cases, so it remained unsupported scientific conclusions.

Its main disadvantage lies in the fact that the exact timing of the renewal of blood is quite difficult to calculate, and sometimes we just don't remember important moments in life that should be taken into account in the theory. Hence, incorrect calculations and incorrect result.
In addition, every body is very individual, all processes occur with different frequency and speed. Someone they are faster, in others it will take much more time. The deadlines for the renewal of blood in men and women too conditional, so faithful to the theory is rather controversial factor.