Even when some of the risk factors subjected to primary prevention may act post-conceptionally (rubella, alcohol, chemical teratogens), their damage is produced so early in embryogenesis that preventive measures are forced to start pre-conceptionally. For this reason, planned pregnancies are at the lowest risk for birth defects. Unfortunately, half of all births come from unintended pregnancies, and they are a major barrier to preconceptional care, now considered as the optimal standard for a better obstetrical and perinatal outcome. Unintended pregnancies include unwanted as well as mistimed pregnancies. North and South American populations differ largely in their proportion of unwanted pregnancies, but not so much in the total frequency of unintended pregnancies Mothers with unwanted pregnancies are more frequently multiparous, of advanced age, and lower educational level.

ACOG (1995) ACOG technical bulletin. Preconceptional care. Number 205. May 1995. Int J Gynec Obstetr 50:201-207.