Chances Of Birth Defects

I'm currently 30 weeks and 6 days pregnant. I have had all of my routine exams and take my prenatals everyday. My ultrasounds look great as well as blood work, NT scan and babies growth. But I'm still worried that my baby will have someting wrong with him. I have 3 healthy children. My husband is healthy as am I.
My SIL has a little girl with Microcephaly. My OB said this should have been detected. It was not picked up during her pregnancy. About two hours after my neice was born, my SIL was told she had numerous things wrong with her. She is now 3 1/2 years ols and has a feeding tube and is in a wheel chair.
What are the chance of something being wrong with my baby and not knowing it until he's born??

[lisaob1]

you will be fine,i promise,we are all different,genetically you have absolutely nothing to worry about,with love and best wishes x

[lisaob1]

you will be fine,i promise,we are all different,genetically you have absolutely nothing to worry about,with love and best wishes x