Ideally you should visit the dentist for a check up if you are planning to become pregnant or as soon as you find out you are pregnant. Your dentist will be able to assess your oral health status and plan any dental visits you may need during your pregnancy.
The initial emphasis will be on a healthy diet and good oral hygiene practices. It is a general convention that as little operative treatment as possible be carried out during pregnancy in order to avoid any possible distress to you or your developing baby. However, it is also accepted that you can have emergency dental treatment at any time.
If you require planned treatment, which is considered essential, the middle of the second semester and the earlier part of the third semester is the time when it will be most comfortable for you. This would be an ideal time to have light treatment like a cleaning carried out.
Click here for more info on maintaining a good oral health routine during your pregnancy.