What do doctors recommend for the spacing between siblings/ between pregnancies?

Let try to explain?: A review of many studies in this area has shown a significantly higher risk of prematurity, low birth weight, and being small for gestational age in babies conceived within six months, compared to babies conceived within 18 to 23 months, of a previous birth. These risks were still evident for babies conceived within 7-17 months of the previous birth.Ihope i was helpfull.Thank.