While the egg cell is still in the ovary, shortly before the ovulation takes place, it has a double chromosome set. Out of this, the first pole body develops which is to be examined in a pole body diagnosis; this pole body has one of the two chromosome sets and is being pushed off for the fertilization so that only the other single chromosome set with the genetic information of the mother remains in the egg cell. An >intracytoplasmic sperm injection (ICSI), the artificial fertilisation of an egg cell by means of injecting a sperm, is a prerequisite for a pole body diagnosis. If the egg cell then fuses with the sperm (which also carries one chromosome set only), an embryo with a double chromosome set develops. The second pole body is pushed off after the sperm has penetrated into the egg cell but still before the fertilisation.
The genetic material that is contained in the pole bodies is excessive; it does, however, form a "mirror image" of the genetic information contained in the egg cell. It can be examined without harming the egg cell and it enables a conclusion as to errors in the distribution of chromosomes that either do not lead to a pregnancy at all or that would cause a miscarriage in the case of a pregnancy. If possible, both pole bodies are being extracted at the same time and the examination result) of the first pole body (which might at that point still be fairly uncertain) is being compared to the one of the second pole body.
According to the German Ethic Council (2003), the reliability of a pole body analysis seems to be comparable to the reliability of an examination of embryonic cells. There are nonetheless constraints as regards the extent of the visible chromosomal derivations: only those anomalies can be caught by means of this method, which were inherited from the mother but not those from the fatherly genome. Furthermore, genetic defects occurring in the later development of the embryo cannot be detected.
In contrast to PGD, the pole body diagnosis was permitted in Germany under very specific circumstances already before the amendment of the German Embryo Protection Act in 2011. For instance, at the time of the pole body extraction, the paternal and the maternal genotype have to be present still separated. The National Ethics Council does hence not consider it "a violation of the embryo protection act to conduct a genetic diagnosis on pole bodies and to abandon egg cells (including the pronuclei)" (2004:1) (unofficial translation). This entails a strong temporal limitation. From the moment of fusion onwards, the embryo protection act applies. According to the prevailing opinion, an extraction of cells is prohibited from that point in time onwards (the District Court of Berlin ruled differently in one individual case. The proceedings are still pending after an appeal by the Public Prosecutor's office).