My advice is to avoid medications were possible in pregnancy as it is not possible to test them in pregnant women so the advice we give is based on observation of those that have been in use for a long time and been shown to be safe. The list below includes medications you can buy over the counter only. Where a prescription is required, the doctor prescribing your medication must be aware that you are pregnant.

Medication which are safe to use include:

IndicationOkay to useAvoid
For painParacetamol and paracetamol with codeineAspirin, Naprosyn, Voltaren
For nauseaVitamin B6, Maxolon 
For thrushCanesten (Clotrimazole) vaginal creamFluconazole oral tablets
For coldsVicks and paracetamolTablets which have Phenylephrine
VitaminsPregnancy multivitaminsVitamin A
AsthmaYour usual medications can continue 
AntacidsMylanta, Omeprazole 
LaxativesMetamucil, Lactulose, Movicol 

Other medications you may already be taking when you become pregnant need to be reviewed as some will need to continue, some changed and some stopped because some can cause problems for your baby. Speaking with your GP prior to pregnancy about any regular medications you take is the best approach.

This website has some useful advice:
Pregnancy Medicines