The reproductive risk arises not in the carrier’s own body but during meiosis the specialised cell division that produces eggs and sperm. In a carrier, the rearranged chromosomes must pair up with their normal counterparts during meiosis, forming a complex, cross-shaped structure. When these chromosomes separate to form individual reproductive cells, several different segregation patterns are possible:

*Alternate segregation: produces reproductive cells that are either completely normal or balanced carriers, like the parent. This is the favourable outcome.
*Adjacent segregation: produces reproductive cells with an unbalanced combination of chromosomal material, containing extra copies of some segments and missing copies of others.
*3:1 segregation (more relevant in certain Robertsonian translocations): produces reproductive cells with an entire extra or missing chromosome.

If a reproductive cell formed by adjacent or 3:1 segregation is involved in fertilisation, the resulting embryo carries an unbalanced translocation. The clinical outcome depends heavily on which chromosomal segments are duplicated or missing and how large those segments are:

*Many unbalanced embryos are not compatible with continued development and result in early pregnancy loss, often before the pregnancy is even clinically recognised.
*Some result in second-trimester miscarriage or stillbirth.
*Others result in a live-born child with intellectual disability, developmental delay, growth abnormalities, or multiple congenital anomalies, depending on the specific chromosomal segments involved.

The overall risk of an unbalanced outcome varies widely from under 5 percent to more than 20 percent per pregnancy, depending on the chromosomes involved, the size of the exchanged segments, and the sex of the carrier parent. This variability is why individualised genetic counseling, rather than general statistics, is essential for each carrier couple.