This hypothesis has received support from studies comparing subfertile couples defined as having a previous treatment or medical visit for infertility or a time to pregnancy of 12 months who conceived without assistance to couples without subfertility; the former group was estimated to have an increased risk of central nervous system defects and NTDs 14, 21, 27, 28 levitra 10 mg bayer J Clin Endocrinol Metab 1984; 59 924 30