As with many other questions concerning marijuana, there is still not solid evidence whether marijuana can lower your chances of conception. This is simply a risk you will have to take on your own, and definitely think about it good whether it is worth it. Some studies suggest that THC, which is the active ingredient in marijuana decreases testosterone levels which is also connected with lower sperm count, and this as many times emphasized lower chances of conception.
Other studies suggest that smoking marijuana can interfere with woman's ovulation and can also disrupt woman's menstrual cycle. The journey of woman's egg through the fallopian tubes after the implantation requires a special environment, a special balance of endogenous chemicals. Unfortunately, even the slightest imbalance of this chemicals can destroy the whole conception process, and since scientist claim THC is a chemical that can affect this hormonal balance, it is very possible that it can also have detrimental effect on the egg during the implantation process and can decrease chances of successful conception.
However, there is still not a solid proof available that could prove that (smoking) marijuana actually affects woman's or man's fertility and chances of conceiving. It is a chance you are taking when you are (ab)using drugs. If you are already having problems with conception, smoking marijuana will probably only decrease the chances of healthy conception. In other words I would say, it is a chance you wouldn't want to take: Call marijuana a drug or a medicine: drugs and babies do not mix.
Several studies have proved that regular use of marijuana causes birth defects, their abnormal development and problems with cognitive capabilities later in life. You actually never know whether marijuana you are smoking is contaminated with herbicides or other drugs, and this my friend puts your unborn child at a huge risk. Would you want to do that to your baby because of (one) joint?