Reduced fertility. Being obese has been shown to be a factor to reduced your chances of getting pregnant. Since being overweight also decreases your health, one should lose as much weight as they can, get healthy, and then get pregnant. You'll have the best chance of getting a healthy baby that way, and less chance of having complications.
Makes response worse. Obesity increases the risk for infertility, miscarriage, and birth defects. It also affects the response to fertility drugs. In general obesity impairs the response to fertility drugs. Take a look at this blog for more detais: http://www.Fertilityauthority.Com/blogger/irms/2011/07/27/fat-biggest-infertility-issue.