Progesterone, the pregnancy hormone, not only relaxes the uterus (womb) in pregnancy, but also other smooth muscle e.g. in the gut. When combined with the pressure of the growing uterus, heartburn (acid regurgitation) is very common. It can be treated with simple antacids such as Rennie, Settlers etc. If the problem persists consult your doctor.
As already mentioned using antacids to neutralize the acid is the first step. You should also try and identify which foods are more likely to cause the problem and avoid these, particularly before going to bed. For severe heartburn we recommend the use of Ranitidine or Zantac, which can be obtained from the chemist.