I think changes in bowel habits are common in pregnancy - changes either way in frequency. If you feel well in yourself and don't have any other symptoms, I wouldn't worry too much. But, of course, do mention it to your midwife just for reassurance.
Without going into too much detail, diarrhoea would be a concern, but frequency is probably just a pregnancy thing.
Edited to add that sometimes frequency can be linked to constipation. So you may be paying lots of visits to the loo because things aren't working quite as smoothly. If that's the case, keep an eye on your water intake to see if a little extra water helps.