There are many causes for delayed child development. Some are neurological.
Some are genetic. Some are orthopedic. Some occur before birth while the child is in utero. Some occur during birth. We always hear about complicated childbirth, and some occur after birth. Some of these diagnoses are genetic or neurological and include diagnoses such as autism, spina bifida, cerebral palsy, Down's syndrome. They can be a host of diagnoses.
What's really important is that you have a trusted medical team that can diagnose correctly your child's condition and then work you to develop the right strategies and interventions that will help your child continue to develop and grow. And above all else, trust your instincts.
You know your child better as a parent better than anyone, and if you feel for any reason that there may be a delay in your child's development, do not delay in seeking help or advice. And by that go speak with your child's pediatrician and have a discussion of your child's development.