This is a difficult question to answer as you don't give much away.
The answers to the questions that follow will help in making a better diagnosis.
1. What colour is your Molly
2. Are there any prutruding scales(scales that look as though they are standing away from the fish)
3. Do you have any other livebearers in the same tank.
4. Can you see the "gravid spot"
5. Is the fish gasping/ hiding away
6. Is the fish still attracting attention from the males
7. is their any "trailing" faeces
8 Does the fish eat more than expected
9 Are there any other unusual signs.
10 What make you think that it's pregnant
Post your livebearer questions in the livebearer forum and you can get more help. If you have both sexes then the female is most likely to be pregnant. She will always be pregnant if that is the case. They can birth every 30-50 days depending on the conditions, the fishes size and age, and temperature.