Having a child with a first cousin raises the odds of birth defects to 6–8% from the norm of about 3–4%.
That can be expressed as either "doubling the risk" or "elevating the risk by three to four percentage points, meaning still more than 90% odds of normal birth"—depending on the spin desired by the speaker.
Having a child with a first cousin raises the odds of birth defects to 6–8% from the norm of about 3–4%.
That can be expressed as either "doubling the risk" or "elevating the risk by three to four percentage points, meaning still more than 90% odds of normal birth"—depending on the spin desired by the speaker.