the depo provera shot affecting my unborn child

I had been on the depo shot for 5 years me and my boyfriend were doing all we could do to prevent an exepected pregnancy but there was a slip and i am now happily 5 1/2 months pregnant with a baby boy, i was not screened for down snydrom so we were wondering if there was a chance that our little boy could have down syndrome seens how i got pregnant on birth control...my OB advised me that my body had most likly became immuned to the shot and thats how our miricale happend. please help us put our minds at ease.

thank you
Allanah