Evidence shows that women who are overweight (or underweight) are less likely to see positive pregnancy test and (finally?) feel the pregnancy signs. I am not saying it is impossible, but if you are overweight and trying to get pregnant, your chances are decreased. It might seem too easy to say woman, lose weight, because I am sure you would already if that would be that easy. But it is not it is easier said than done. First thing comes first.
If you are overweight and trying to get pregnant but you can't, you should try to lose weight first. We have established that before and if you had tried to lose weight, including cutting calories, eating healthier in terms of eating more proteins, less carbs, more healthy fats, and most of all in terms of including moderate exercise in your daily routine, but your goal was not achieved or you were losing weight slower than expected, it is time to visit your doctor. There are many health issues that might be preventing you from losing weight: perhaps you have issues with blood sugar or you might be diabetic but you aren't aware of that. Are you feeling tired often? Maybe you are suffering from PCOS which additionally causes problems when woman is trying to get pregnant. Perhaps you have underactive (overactive) thyroid gland?
These and many more are the questions your doctor should be able to answer with the help of some blood and other tests. Therefore, if you are experiencing problems with losing weight and you are really doing your best to lose weight, but the air that you breathe makes you gain it all back, it is time to visit your G.P. However, if you are starving yourself and still forgetting the principles of good nutrition, doctor can't help you if you don't want to help yourself. Including quality nutrition and moderate exercise in your daily routine is the key to healthier you.