When Is a Baby’s Sex Determined? A Must-Read for Intended Parents Considering Surrogacy in Mexico

When intended parents begin exploring IVF and surrogacy in Mexico, embryo quality, transfer timing, and the pregnancy process are often key concerns, but the sex of the embryo is also something many people want to understand. Especially when third-party assisted reproduction is involved, many people have a very direct question: When is a baby’s sex actually determined? Is it at fertilization, or only after the embryo develops?

Sex Is Determined at Fertilization

From a biological perspective, chromosomal sex is determined when the sperm and egg combine to form a fertilized egg.

A woman’s egg typically carries an X chromosome, while a man’s sperm may carry either an X chromosome or a Y chromosome. When an X-carrying sperm fertilizes the egg, the resulting embryo is typically XX; when a Y-carrying sperm fertilizes the egg, the embryo is typically XY.

Therefore, the embryo’s chromosomal sex is not determined after the embryo is transferred into the surrogate’s uterus, nor is it determined by the surrogate. In surrogacy in Mexico, the surrogate carries the pregnancy and gives birth, while the genetic information of the embryo comes from the egg and sperm used to create the embryo.

When Can the Sex Be Known During Embryo Development?

Although the sex chromosomes are determined at fertilization, this does not mean they can be observed with the naked eye as soon as the embryo forms.

After fertilization, the embryo undergoes continuous cell division and gradually develops from an early-stage embryo into a blastocyst. If IVF-PGT, namely IVF-PGT embryo genetic testing, is performed, the embryo’s chromosomal status can be analyzed at the blastocyst stage.

Among the information obtained through testing, information related to the X and Y chromosomes may also be reflected in the results. Therefore, for families undergoing IVF-PGT, sex-related information is obtained during the embryo genetic testing stage rather than being determined after embryo transfer.

It should be noted that the primary value of IVF-PGT is to provide information about the embryo’s chromosomes and genetics. The specific information that can be tested and reported depends on the testing method used as well as local laws and regulations.

Different countries and regions have different regulations regarding assisted reproduction, embryo genetic testing, and sex selection for non-medical reasons. Even when such information can technically be obtained, how it may actually be used needs to be determined based on local laws and the specific policies of the medical institution.

For those who are seriously planning their reproductive future, understanding “when is sex determined” is only the first step in understanding embryo development. Once IVF and surrogacy in Mexico become part of the reproductive plan, it is also necessary to clarify the available eggs, sperm, embryos, and IVF-PGT testing arrangements step by step. POWER FERTILITY CENTER can serve as a starting point for learning more about related reproductive medical information.

Power Fertility Clinic is a fully licensed reproductive medical institution in Mexico that follows U.S. FDA standards and is directly operated by a U.S. assisted reproductive services organization. Located in central Mexico City, just 500 meters from the U.S. Embassy, it is equipped with advanced laboratories and a professional team and provides comprehensive reproductive medical clinical services. Power IVF covers egg donation, sperm donation, third-party reproductive services, as well as egg freezing, IVF, and embryo screening (PGT-A). In Mexico, IVF, sex selection, and third-party reproductive services are legally available to all families, including single individuals, married couples, and LGBTQ groups, and regulations support the importation of cryopreserved reproductive tissues, including sperm, eggs, and embryos. Power IVF is licensed to receive cryopreserved reproductive tissues from overseas, providing clients with efficient and professional reproductive solutions.