In Vitro Fertilization (IVF) is a medical procedure that helps prospective parents conceive by fertilizing eggs with sperm outside the body. This process, along with other assisted reproductive technologies (ART), has become a common treatment for infertility worldwide. A frequent question that arises among individuals considering IVF is whether the procedure influences the sex of the resulting child. Examining large-scale data and underlying biological mechanisms is necessary to determine if IVF naturally results in a skewed sex ratio compared to natural conception.
Observed Sex Ratios Following IVF
The natural sex ratio at birth in human populations, also known as the secondary sex ratio, typically favors males slightly, with approximately 105 to 106 male births for every 100 female births. This ratio translates to roughly 51.5% of newborns being male. Large-scale studies and meta-analyses consistently suggest that IVF procedures tend to amplify this slight male bias, although the difference is minor.
The general consensus is that births resulting from standard IVF cycles show a small, statistically significant increase in the proportion of males compared to naturally conceived children. For instance, some cohorts have reported an overall male birth rate of around 52.1% following ART. This small shift is not the result of intentional selection but rather an inadvertent outcome of the laboratory environment and procedures used.
Biological Mechanisms Influencing Sex Ratio
The observed male skew appears to be a fundamental difference in the biology and metabolism of male (XY) and female (XX) embryos. Differences in genetic expression, specifically on the Y-chromosome, influence developmental speed directly after fertilization. Male embryos often exhibit a slightly faster rate of growth and development than their female counterparts during the early preimplantation stages in the lab.
This difference is partially due to the metabolic requirements of the embryos. Female embryos, which possess two X chromosomes, have been shown to have a higher consumption of glucose early in development. Conversely, male embryos may have a metabolic advantage in the specific chemical environment of the culture media. The preferential development of male embryos means they are statistically more likely to reach the desired developmental stage, such as the blastocyst, faster. This differential growth rate can then lead to their favored selection.
Impact of Specific Assisted Reproductive Techniques
The stage at which the embryo is transferred influences the resulting sex ratio. Blastocyst transfer (Day 5 or 6) is consistently associated with a higher proportion of male births compared to earlier cleavage-stage transfer (Day 3). This procedural preference reinforces the selection of the faster-developing male embryos.
The specific formulation of the culture media used to grow the embryos also plays a role in creating a sex skew. Studies have demonstrated that different commercial culture media can lead to variations in the male-to-female birth ratio. This effect is likely due to how the media’s ingredients selectively impact the developmental kinetics of male embryos, favoring their survival and growth. Intracytoplasmic Sperm Injection (ICSI), a technique where a single sperm is injected directly into the egg, also affects the sex ratio, often resulting in a slightly lower proportion of males compared to conventional IVF.
Distinguishing Natural Skew from Intentional Selection
It is important to differentiate the small, naturally occurring statistical skew from intentional sex selection. The natural male bias observed after IVF is an unintended consequence of the laboratory environment. Intentional sex determination is achieved through Preimplantation Genetic Testing (PGT), which accurately identifies the sex of the embryo before transfer.
PGT is primarily used to screen embryos for genetic or chromosomal abnormalities, but it can also reveal the embryo’s sex. Using PGT solely for non-medical sex selection is a regulated practice. Many jurisdictions worldwide, including most of Europe, Canada, and India, legally prohibit the use of PGT for elective sex selection. In the United States, while the practice is legal, major medical organizations urge caution due to ethical considerations and societal gender bias.