What is the best age gap between first child and second child?

Women ideally require a minimum of 18 months after childbirth before having another child. Closely spaced pregnancies are linked with increased risks of preterm births, placenta abruption, placenta previa and autism. Therefore, 18 months + 9 months = 27 months. The closest age gap you should have for your children is 27 months. http://www.mayoclinic.org/healthy-lifestyle/getting-pregnant/in-depth/family-planning/art-20044072