Aten is the brand name of a medicine called atenolol. It belongs to a class of drugs called betablockers. Aten, like other betablockers, should not be used in pregnancy because the drug reaches the fetus via mother's blood and can cause side effects. There is no data/experience with its use in early pregnancy since it is contraindicated but when used in mid and late pregnancy even under close medical supervision, there have been cases of harm to the fetus.
The only safe drug during pregnancy is alpha methyldopa sold under the brand name of Alphadopa. Its dose is 250 to 500mg 2-3 times daily.