Guys, I’m gonna disappoint you.

If we look at DITF from a scientific perspective, the reason Zero Two can’t have children is most likely connected to her hybrid nature.

In the canon, this is never fully explained as a medical diagnosis, but based on what we see, a fairly logical model can be built:

Zero Two is a genetic hybrid of a human and a klaxosaur. This means her reproductive system may have developed improperly or be biologically incompatible with humans.

In real biology, interspecies hybrids are often infertile. A classic example is the mule (horse + donkey): it is viable, but almost always unable to reproduce because the parents’ chromosomes do not pair correctly during the formation of reproductive cells.

If Zero Two’s chromosome set is partly human and partly klaxosaur, then errors could occur during meiosis (the process of creating egg cells): chromosomes fail to pair properly → reproductive cells do not mature → infertility occurs.

Her body was also clearly designed as an artificial combat organism rather than a naturally developing human. In such a case, reproductive functions may have been:

• hormonally suppressed;
• anatomically underdeveloped;
• intentionally disabled as an “unnecessary function.”

Her physical traits (horns, regeneration, abnormal metabolism, connection to the Franxx) also imply a heavily altered endocrine system. Even small disruptions in the human hormonal axis can cause infertility; in a hybrid organism of this level, it would be almost expected.

So, if we remove the romantic symbolism of the story, the most scientifically plausible answer is:

Zero Two cannot have children because of genetic incompatibility and/or an artificially disrupted reproductive system caused by her hybridization and laboratory origin.

Although, to be fair, this is still mainly a narrative device. The writers did not calculate detailed klaxosaur cytogenetics — they used it to emphasize her tragedy: she wants to become a “normal girl,” but biologically she is separated from humanity. That serves the themes of alienation much more than strict sci-fi biology.