BH my wife is pregnant. How do we calculate Onot?
You should calculate the onot regularly until she passes them for the first time. Meaning that she should check herself on the onah benonis and yom hachodesh, (which probably passed by the time you find out that she is pregnant), and the haflagah. After these pass she doesn’t have to check for the next yom hachodesh.
BT”W it is common for women to have light staining during pregnancy at the time that the first period should of come. If this happens she should count the regular onot.
If for whatever reason there would be an onah after three months from the mikvah, then she does not have to count that at all.
Y:D 184-7Chasam Sofer ibid 69, Shiurei Shevet Halevi ibid 1.