Gender Selection: Why & How It’s Done

Genetic testing to screen out defective embryos before implantation in IVF also reveals gender, giving parents a choice of having a boy or girl.

History of gender selection

Preimplantation genetic testing was initially developed in England in 1990 to detect single-gene and sex-chromosome linked diseases in at-risk couples. The test then was adapted as part of in vitro fertilization (IVF) to detect abnormal chromosomes in infertility patients. Use of this amazing technology is growing every year.

A few years ago, using the technology available, it was only possible to test up to five pairs of chromosomes in one embryo. Now, all chromosomes can be tested in a dramatic breakthrough, which enables us not only to determine the sex of the embryos for people interested in gender selection but also to lower the miscarriage risk. It also helps to eliminate the possibility of conditions such as Down syndrome, caused by three copies (trisomy) of chromosome pair # 21. The scientific analysis of the chromosomes is carried out at a national reference laboratory with the required skill and equipment to perform preimplantation genetic testing.

There are two forms of preimplantation genetic testing: PGS (preimplantation genetic screening) and PGD (preimplantation genetic diagnosis). PGS evaluates the chromosomal numbers and sex, while PGD evaluates for individual specific inheritable disorders. The test used for gender selection is PGS.

How it works: The science

First, when someone decides to do genetic testing on embryos, IVF is done and embryos develop to the fifth day. The embryos are biopsied and frozen. PGS evaluates all 24 pairs of chromosomes. The chromosomal material is expanded and the number of chromosomes is counted. The sex chromosomes are either XX in a female or XY in a male. The gender of each embryo is discovered and the genetically healthy embryos are identified.

At least half of the embryos in many cases are aneuploid, which means they have abnormal numbers of chromosomes. Aneuploidy arises from errors in the production of chromosomes by the egg before the sperm arrives. There should be pairs of chromosomes. Common aneuploidy examples are trisomy (triple chromosome) or monosomy (single chromosome). In either of these cases, if an implantation occurs from such an embryo, we would expect either a miscarriage or an abnormal offspring.

The diagnosis is correct usually 90 percent of the time, but there can be times when the chromosomal material is not reflective of the actual outcome. This is not rare; estimates vary from 2 percent to 16 percent.

Who should consider preimplantation genetic testing?

Couples who experience the following situations should consider having a preimplantation genetic evaluation of their embryos:

  • Recurrent miscarriage, with chromosomal abnormality either detected or suspected
  • Repeated IVF failure, in which healthy looking embryos were implanted
  • A family history of a recessive or dominant genetic error related to a specific disease or genetic disorder
  • A desire for gender selection.

In certain situations, patients would like to choose the gender of their baby. This is a request based on personal, family and cultural values. For the most part, couples that pursue trying to have a boy or a girl for family balancing are fertile and do well with IVF. In the United States, a survey indicated that only 8 percent of the population would be interested in considering prenatal gender selection, and this interest related predominantly to family balancing (Dahle et al.).

The genetic testing process 

The couple should be screened for their fertility potential, including looking at the ovaries with an ultrasound and testing the anti-müllerian hormone (AMH), which is a predictor of ovarian reserve. Some young women are surprised to find that they have relatively low reserve, which impacts the number of eggs that will be retrieved.

Powerful gonadotropin hormones are used to stimulate the development of eggs. Three to five ultrasound and blood test combinations are used to follow the progress of the development of the follicles. Then IVF is carried out, which includes an egg retrieval procedure by transvaginal ultrasound guidance done under anesthesia. The father’s sperm is used the same day to fertilize the eggs.

On day 5, an embryologist biopsies the barely visible embryos, called blastocysts, with a micromanipulator. The genetic scientists can take this infinitesimally small bit of tissue and expand the DNA to determine the chromosomal compliment of the embryo, as well as its gender.

One other gender selection option

Sperm selection by cell sorting, or MicroSort, can be used to improve the number of sperm that are X or Y, depending on whether the couple is trying to have a boy or a girl in their family balancing plans. Since this procedure is not available in the United States, this sperm must be shipped to Mexico at an additional cost. The accuracy is estimated at 80 percent for female gender selection and 70 percent for male selection.

The myths of gender selection

It is comforting to know that the science of PGS for gender selection can replace myths about how doing certain things can affect conceiving a boy or a girl.


  • Intercourse position & time determines the gender. It is a myth that having intercourse in various positions, different times in the month, or according to the day of ovulation can determine the baby’s gender. These are simply not reliable at all for people who are interested in family balancing.
  • What you eat affects gender. Folk wisdom holds that if you want a boy, eat meat, red meat in particular. If you want a girl, eat more vegetables and fish, as well as sweets and chocolate. This is not proven.
  • Gradient method of sperm washing. Some physicians are offering the gradient method of washing sperm, proposing to increase the number of boys or girls according to the wishes of the parents. They claim that centrifugation helps separate the heavier X-bearing sperm away from the male Y-bearing sperm. This is not validated scientifically. Very few reputable clinics worldwide are proposing this method. Patients should question the statistics from anyone offering this method.



  1. American Society of Reproduction Medicine. 2001.
  2. Dahle et al. Preconception Sex Selection Demand and Preferences in the United States. Fertile Sterile. 2006; 85: 468–73.
  3. Ethics Committee of Preconception Gender Selection for Nonmedical Reasons. Fertile Sterile. 2001; 75: 861–864
  4. Ethics Committee of the American Society for Reproductive Medicine. Fertile Sterile. 2015; 103: 1418–1422.
  5. Robertson, J.A. Extending Preimplantation Genetic Diagnosis: The Ethical Debate. Human Reproduction, 2003; 18: 465–71.