I'll have to do some digging but I believe a study done by John Hopkins put to rest this old theory and it's actually all fetuses have both. Just a little search produces this.
; all embryos, regardless of their sex, initially develop structures that have the potential to become either male or female reproductive tracts, meaning they essentially have "both" male and female reproductive tracts in their early development stages before differentiation occurs based on genetic factors.
Explanation:
Undifferentiated Stage:
Early in development, the gonads in an embryo are undifferentiated, meaning they can develop into either testes (male) or ovaries (female) depending on the presence of the Y chromosome.
Müllerian and Wolffian Ducts:
Both male and female embryos possess two sets of ducts called the Müllerian ducts and Wolffian ducts, which are the precursors to the internal reproductive organs.
Male Development:
In a male embryo, the presence of the Y chromosome triggers the development of the testes, which then produce testosterone causing the Wolffian ducts to develop into the male reproductive tract while the Müllerian ducts regress.
Female Development:
In a female embryo, lacking the Y chromosome, the Müllerian ducts develop into the female reproductive tract while the Wolffian ducts naturally degenerate.
Key points to remember:
All embryos start with the potential for both male and female reproductive organs.
The presence of the Y chromosome determines the development of male genitalia.
In the absence of a Y chromosome, the embryo will develop female genitalia by default.