fibre supplements such as bran, ispaghula and sterculia are safe.

http://www.patient.co.uk/health/planning-to-become-pregnant

If this fails then fibre supplements such as bran, ispaghula and sterculia are safe. If you need something stronger then it is best to discuss this with a doctor. Some laxatives such as docusate and lactulose may be prescribed safely for a short time.