Excellent question truthseeker!
Nothing to worry about.....Unlike both medical and psychiatric conditions which are definitely hereditary, developmental disorders are not. They are essentially caused by external issues/factors such as the age the mother (over 35, or even father over 50) at the time of conception, which may cause a random genetic defect, or something during the birthing process, where perhaps the newborn is oxygen deprieved. Development Disorders may also may be caused by the mother drinking alcohol during her pregnancy (fetal alcohol syndrome), or even heavily smoking.
Don't worry. The baby, when it comes, will have a full and happy life!
DDs are not hereditary like cancer, heart disease, depression, or schizophrenia. Most often external causes lead to it. Autism may have some genetic link, but generally it is felt that perhaps some unknown virus or perhaps overuse of vaccines may play into its development.
If the mother to be has more than one DD person in her nuclear/extended family, then be concerned, if not.... don't worry. The baby will be fine.
Because if there is more than one, that would be an indicator of a family history or hereditary nature of the DD. Ask about grandparents,aunts, uncles, cousins, second counsins, etc... I bet you will find nothing. This applies to all DDs, even Asbergers, which is a very mild form of autism. With mental illness, 50% of my patients have somebody in their immediate/extended family who also has the disorder.