On the (im)possibility of self-fertile human hermaphrodites
I recently read a rather unusual article, discussing the possibility that certain humans may be able to conceive and bear children completely on their own. The idea is that in certain rare cases, a person may have both an ovary and a testis, producing eggs and sperm, and that these may then form embryos that develop to babies.
As a reproductive biologist, I’d like to give my perspective on why this is exceptionally unlikely and has probably never happened.
A question of parts
To make an embryo, a sperm (from a testis) needs to encounter an egg (from an ovary). Additionally, for the embryo to develop into a baby, it needs to implant in something which can support its growth. Ideally this is a uterus, but in very rare cases, ectopic pregnancies have resulted in live babies delivered by C-section.1
An ovary requires two X chromosomes to be fully functional, whereas a testis requires an X and a Y chromosome. There have been cases of XX testes or XY ovaries in humans due to rare mutations, but in all these cases, no viable sperm or eggs are produced due to problems with meiosis.2 Therefore, in order to make both eggs and sperm, a person must have a mixed XX / XY karyotype. This is possible, and cases have been reported in the medical literature, but very rare.
And as HedonicEscalator correctly states, it’s not as simple as just making a sperm and an egg:
among other things, it also requires a fully functioning uterus. Not impossible, but improbable enough that it might have never happened.
I would also add that the sperm and the egg need to meet each other. In biological females, the egg travels from the ovary down the Fallopian tube to the uterus, where it implants after fertilization. In males, the sperm travel through the epididymis and vas deferens. If those tubes are missing, the sperm can’t leave the testis, and the result is male infertility.
Battle of hormones
However, these male and female structures can’t coexist during normal reproductive development. The fetal testis produces anti-Mullerian hormone (AMH), which suppresses the development of the Fallopian tubes and uterus, and testosterone, which is necessary for the development of the epididymis and vas deferens. AMH has a relatively short range of diffusion during fetal development, so an individual with a single testis on one side of their body will sometimes have a Fallopian tube on the other side, but in such cases there are usually uterine abnormalities. In extremely rare cases, mutations in AMH or its receptor can cause the presence of a uterus in individuals with testes, but these mutations also tend to decrease male fertility.
Furthermore, the testosterone produced by the testis would act on the ovary during fetal development, likely causing something similar to polycystic ovarian syndrome. This would not cause complete sterility, but would reduce the frequency of ovulation.
Finally, after puberty, both the ovary and the testis would be producing their respective sex hormones. In order for ovulation to occur, levels of reproductive hormones need to cycle over time, but this may not happen properly if a testis is present.3 However, about 1/3 of trans men who take male sex hormones do occasionally ovulate in spite of this, so this is not a complete block.
Doing the numbers:
My estimates (note, these numbers are very rough):
Incidence of XX/XY chimerism with both ovary and testis formation:4 still unknown (according to studies in 2020 and in 2022). The study in 2022 reported “at least 50” cases in medical literature since 1962. As a rough estimate, I assume there are 100 births per year and most go undiagnosed.
Chance of testis and ovary being able to produce both sperm and eggs in such cases: 20% (20 cases per year)
Scenario 1: Ectopic pregnancy5 leading to live birth:
Chance of sperm and eggs finding each other without the presence of a Fallopian tube or uterus: 1%
Chance of ectopic pregnancy leading to live birth: 0.1% (and 0% before 1900)
Total estimated prevalence: 1 per 5000 years (and only possible by C-section)
Scenario 2: Persistent Müllerian duct syndrome (full uterus formation in presence of a testis):
Also very rare! Less than 300 cases worldwide from when the condition was first described in 1939 up to 2022. Some may go undiagnosed, but even accounting for this, the rate is unlikely to be above 1 per million people.
Chance that egg and sperm find each other: 50% (the Fallopian tube is present, but there may not be a path for the sperm to get out of the epididymis and vas deferens).
Chance that uterus is actually functional: 50% (sometimes it’s only partially present).
Total estimated prevalence: 1 per 200,000 years
Overall, self-fertile human hermaphrodites aren’t biologically impossible,6 but it’s not surprising that we haven’t seen any.
Usually ectopic pregnancies cannot survive to term, and I wonder if those that do survive are actually a result of endometriosis, where uterine tissue grows outside the uterus. (Endometriosis is a known risk factor for ectopic pregnancy.)
I am talking about humans here. Some other species are more tolerant of incorrect sex chromosomes during meiosis.
FSH and LH from the pituitary are particularly important. Hormones produced by the ovaries and testes have feedback effects on the pituitary, and in males, there aren’t huge periodic spikes of FSH and LH the same way there are in females.
The incidence of XX/XY chimerism itself is likely higher, but in most chimeras, the reproductive organs will not be involved. If someone’s liver is XY whereas the rest of their body is XX, they probably won’t notice.
This scenario includes cases where there is some uterine tissue due to incomplete action of AMH. These cases would be functionally similar to an ectopic pregnancy (requiring a C-section), because a half-formed uterus likely couldn’t support a natural delivery.
Certain fish can do this, although they don’t have a uterus and the embryos develop externally.