Do All Babies Start Female?
No, not all babies start female. This is a common misconception stemming from the fact that early fetal development shows similarities in genitalia before sex differentiation.
While it's true that early in development, the genitalia of a fetus are initially undifferentiated and phenotypically female (meaning they appear female), this does not mean all fetuses start as female. The key is that the development is undifferentiated, not definitively female. The sex of the embryo is determined at fertilization by the sex chromosome carried by the sperm.
- XX Chromosomes: If the sperm carries an X chromosome, the resulting embryo will have XX chromosomes and develop as female.
- XY Chromosomes: If the sperm carries a Y chromosome, the resulting embryo will have XY chromosomes. The presence of the Y chromosome triggers the development of testes, leading to male characteristics.
The common misunderstanding arises from the initial similar appearance of genitalia before approximately 6-7 weeks of gestation. After this period, the presence or absence of the Y chromosome and its gene expression initiates the differentiation into male or female characteristics. This is a crucial point often overlooked.
Several sources highlight this:
- Invita: "No, that is not true. The sex of humans is determined from fertilization, that is, as soon as it becomes a zygote: if it is XX, it will be a female, and if it is XY it will be a male." (https://www.invitra.com/en/faqs/)
- Pampers: "Do all babies start as female? No, but we can understand why this is a common question, as all eggs in a female body have an X chromosome." (https://www.pampers.com/en-us/pregnancy/pregnancy-announcement/article/what-determines-the-sex-of-a-baby)
- Genetic Literacy Project: "Embryos aren't female by 'default' after all, study shows ... Every developing embryo, irrespective of its sex, at one point contains both male and female characteristics." (https://geneticliteracyproject.org/2017/09/21/embryos-arent-female-default-study-shows/)
The initial similarity in fetal genitalia is a stage in development, not an indication of an initial female state. The genetic makeup at fertilization determines the sex of the baby.
