In vitro fertilization (IVF) is a widely used and effective fertility treatment that assists individuals and couples in achieving pregnancy. This process involves retrieving eggs, fertilizing them with sperm in a laboratory, and then transferring the resulting embryos into the uterus. As IVF becomes more common, questions arise about its potential influence on the sex of offspring. This article explores what research reveals about sex ratios in IVF.
Biological Basis of Sex
Human biological sex is determined by specific chromosomes inherited from parents. Females typically have two X chromosomes (XX), while males possess one X and one Y chromosome (XY). During natural conception, an egg always contributes an X chromosome. The sperm, however, can carry either an X or a Y chromosome, making it the determinant of the offspring’s sex. If an X-carrying sperm fertilizes the egg, the result is a female (XX) embryo; if a Y-carrying sperm fertilizes the egg, a male (XY) embryo develops. In natural conception, the chance of conceiving a male or female is approximately 50/50, though the global sex ratio at birth shows a slight male bias, typically around 1.05 males for every female.
Research on IVF and Observed Sex Ratios
Scientific studies investigating the sex ratio of babies conceived through IVF present a nuanced picture, often showing some deviation from the natural 50/50 ratio. While the overall sex ratio for assisted reproductive technology (ART) in some regions, like the UK, closely mirrors natural conception, specific IVF procedures can influence the likelihood of conceiving a male or female. For instance, a large-scale study in the UK found an overall sex ratio of 104.0 males per 100 female births for ART, which is comparable to the natural ratio of 105.3 males per 100 female births in England and Wales during the same period.
However, different fertilization methods show varying trends. Conventional IVF, where sperm and eggs are mixed in a dish, has been associated with a slightly higher proportion of male births, sometimes reaching 110 males per 100 female births. In contrast, Intracytoplasmic Sperm Injection (ICSI), where a single sperm is injected directly into an egg, has shown a tendency towards more female births, with some studies reporting ratios as low as 97.8 males per 100 female births, or even 50% girls.
The stage at which embryos are transferred to the uterus also appears to play a role. Transferring embryos at the blastocyst stage, typically around five to six days after fertilization, has been linked to a higher proportion of male births, with reported male percentages ranging from 54.1% to 59.5%. Conversely, transferring embryos at the earlier cleavage stage, usually two to three days after fertilization, tends to result in a more balanced sex ratio, or even a slight female bias, with male percentages reported between 49.9% and 51.4%.
Factors Potentially Influencing Sex Ratios in IVF
Several factors within the IVF process are hypothesized to influence the sex ratio of resulting offspring. The composition of the culture medium, the specific chemical environment where embryos develop in the laboratory, is one such area of investigation. Research suggests that conditions in the culture medium might subtly favor the development or survival of embryos of one sex over the other. One theory indicates that impaired inactivation of the X chromosome in female embryos under IVF conditions could lead to developmental issues or even lethality, thereby skewing the sex ratio towards males.
The timing of embryo transfer is another factor that appears to correlate with sex ratios. Male embryos are often observed to grow and develop faster than female embryos, potentially reaching the blastocyst stage earlier. When embryologists select the most morphologically advanced embryos for transfer at the blastocyst stage, this natural difference in developmental speed might indirectly lead to a higher proportion of male embryos being chosen and transferred. This could explain the male bias seen in blastocyst transfers compared to earlier cleavage-stage transfers.
Preimplantation Genetic Testing (PGT), particularly PGT for aneuploidy (PGT-A), can directly determine the sex of an embryo by screening its chromosomes. This testing allows for the identification of XX (female) or XY (male) embryos before transfer. While PGT is primarily used to screen for chromosomal abnormalities and genetic diseases, it can also be employed for intentional sex selection, whether for medical reasons, such as preventing the inheritance of sex-linked disorders, or for family balancing.