Most parents would be happy to have an infant regardless of gender, boy or girl. However, for various reasons, some strongly prefer one gender over the other. We see this arise, for example, for cultural reasons or when a family already has many children of one gender and simply wants to achieve gender diversity.
This is not new. People tried to control the gender of their conceptions long before IVF came along. For example, some believe that intercourse in a certain way will control the gender of the offspring. This idea seems to be supported by the fact that the X (female) or Y (male) chromosome from the sperm determines the sex of the embryo (the egg or oocyte always carries an X chromosome). Because the Y chromosome is smaller and therefore lighter than the X chromosome, some believe that this allows sperm carrying the Y chromosome to swim faster. In practice, this difference is too small to matter, and such techniques do not produce substantially more infants of either sex than would otherwise be expected. Sex-selection techniques based on differences in sperm swimming speed are not very effective.
With IVF, however, we have the ability to examine the genetic makeup of an embryo. We can screen embryos in culture for chromosomally-carried diseases (single-gene defects), such as cystic fibrosis, for example. Only those embryos that do not have the disease-carrying gene are then transferred to the uterus. We can also check the number of chromosomes in each embryo and transfer only those with the normal number. It is hoped that this approach will increase IVF success rates per transfer by eliminating the transfer of non-viable embryos. These genetic tests are commonly known as PGD or PGS (preimplantation genetic diagnosis or screening).
PGD and PGS also reveal the sex of the embryo, allowing sex selection. Female fetuses have two X chromosomes (one from each parent) while male fetuses have one X chromosome from the mother and one Y chromosome from the father. Transferring only those embryos of one sex will greatly increase the chance that the resulting children will be of that sex. This is a very effective technique for gender selection.
To learn more about gender selection, contact the Center for Reproductive Medicine and Fetal Medicine – Dr. Alexander Trajan for a consultation.