[KNDS]

In nature, usually majority of zygotes (fertilized eggs by a sperm) are in fact aborted and miscarried in the menstruation process or in a new separate event called miscarriage. The genetic cause for these are chromosomal disorders: incorrect chromosome number due to deletions, duplications and many other abnormal results of cell divisions in the sex cells (egg or sperm) or in the newly created zygote.
I think in these states, the Gandhabba is unsuccessful in gettin into the “zygote” as the zygote does not provide a suitable environment to create a viable human being.