It's a boy or a girl? - sex determination and ART influence on secondary sex ratio

• Published in: Reproductive biomedicine online 2024-05, Vol.48, p.104059, Article 104059
• Main Authors: Costa, L., Pires, I., Cabral, M., Vieira, M., Costa, S., Silva, F., Pinelo, S., Serra, H., Barbosa, A., Felgueira, E.
• Format: Article
• Language: English
• ISSN: 1472-6483
• DOI: 10.1016/j.rbmo.2024.104059

There are some assumptions about sex determination in humans. It is known that the sex of the baby is determined by the sperm. However, a disproportionate loss of male is well documented after conception (primary sex ratio) and at birth (secondary sex ratio, SSR), reaching balance only in the third or fourth decade of life (tertiary sex ratio) and declining with age. At 16-19 weeks of gestation, the SSR is 2.48 (248 boys for 100 girls), decreasing to 1.30 at 20-36 weeks, reaching 1.05 at birth. It is not clear yet why SSR varies during the different stages, but it is known that male gender is an independent risk factor for adverse perinatal outcomes. It has been suggested that sperm are smaller, faster, have a shorter life span and prefer more alkaline environment. Accordingly to such observations, methods for conceiving boys or girls were developed, which had a questionable scientific basis. Assisted Reproductive Techniques (ART) are increasingly contributing for the number of children born around the world, contradicting natural selection. Therefore, the impact of ART on sex determination is also questioned. In this work, a critical review was carried out on the factors that could influence the sex of the baby born after ART, comparing it with the data from our IVF Unit. The overall calculated SSR was 1.13 (n=2347). According to what is described, the SSR after IVF was higher than ICSI (1.24 vs 0.99). Transfer of blastocyst also appears to favor SSR when compared to cleavage embryo transfer (SSR=1.27 vs 1.06). Contrary to what has been reported, cryopreserved embryo transfer resulted in a decrease in SSR, regardless of the stage of embryonic development (SSR= 0.95 at day-3, 1.10 at day-5). Woman's age has been negatively correlated with SSR, a theory not confirmed by our data. On the other hand, the existence of a male factor appears to have little impact on SSR. Although ART seeks to optimize results to contribute to the birth of more and more children, the impact of these techniques on the natural balance of life should not be ignored.