It depends, will it be harder to be pregnant right away with a younger child, or pregnant with an older child but you yourself are also older? There are similar risks between waiting less than 18 months and waiting 5 years or more between pregnancies. Also, new studies show the risks of a 12 month wait were basically equivalent to those at 18 months. There are also different risks for being pregnant after 35. I was cleared at 9 months pp to start TTC (internal/external ultrasounds, etc.).