Birth defects - sometimes severe - occur at a rate of roughly 1 per 33 births worldwide. Which currently equates to nearly 8 million per year.

Of these, approximately 20% can be accounted for (genetic defects, smoking, viruses and bacteria, obesity, stress etc) the remaining 80% have unknown causes.

The figures come from a five-year large-scale 2017 study :

The overall conclusion remains that a specific cause cannot yet be determined for most birth defects, underscoring the current gaps in knowledge and the challenge of primary prevention.

Source : Etiology and clinical presentation of birth defects: population based study BMJ 2017;357:j2249