"The cough expectorant guaifenesin to help loosen a chesty cough can be used, though drinking lots of water is just as effective for this. You could also try using steam inhalations to help liquify mucus and make it easier to cough up. Cough medicines containing iodine as an expectorant should be avoided, as the iodine can impair the functioning of the baby's thyroid gland.
The cough suppressant dextromethorphan has been in widespread use and is generally considered safe to use during pregnancy, providing it is only taken for short periods of time and using the smallest dose possible. However, it is probably best to avoid it in the first trimester. Cough suppressants containing codeine should be avoided in the third trimester."
this may help.
taken from
http://www.netdoctor.co.uk/health_advice/facts /medicinesinpregnancy.htm