There are some medications that are definite "no-nos" when you're trying to conceive because they are dangerous to a developing baby.
I hope everybody knows that Accutane is one of the worst medications you can be taking when you get pregnant or are pregnant. This is so serious that doctors can only prescribe it for a month at a time, and women have to have a negative pregnancy test before they can start on it. While taking it, women must also be using some form of birth control. It leads to serious birth defects. I once filled a prescription for a dog who had folliculitis, and even the veterinarian was not allowed to prescribe more than a 30-day supply.
Another type of medication that women should not be taking when they're trying to get pregnant is the "statins" for high cholesterol. These includes drugs like Zocor, Lipitor, Crestor, and Vytorin. These drugs block the production of cholesterol, but that action is potentially dangerous to a developing fetus. Babies need cholesterol--cholesterol is an essential component of every cell wall in our bodies.
If you have high cholesterol and want to get pregnant, you should stop taking "statin" medications. There aren't any that are safer than others.
Drugs for high blood pressure can also be risky, especially the ACE-inhibitors like lisinopril and captopril. These cause birth defects when taken in the 2nd and 3rd trimester, but I would be wary of taking it earlier, too, as there are some reports of birth defects when taken earlier. There are lots of medications for hypertension, so talk to your doctor about an alternative.
If you have UTI or bacterial vaginosis, don't use metronidazole if you're trying to conceive or are pregnant. It could cause a miscarriage or possibly increase the risk of cancer. Unless you're absolutely positive you're not pregnant, there are alternatives you can take instead.
Topical medications may also have some risks, like Retin-A. The key is that you do not know for sure that you're pregnant for at least a couple weeks, and who wants to look back and worry about what you've been doing during that time? Make your medication and lifestyle changes before you even try for conception so that you can sleep easier once you are pregnant.