I've said this before in another post, but there are cases where women don't realize they are pregnant until long into the pregnancy. There are symptoms and cases that are typical, but atypical cases are not very rare at all. If she did in fact have an IUD, it is possible that the menstrual cycles take a while before becoming regular (and in some documented cases, pregnant women have been known to menstruate through out pregnancy). And the stress she's been throughout this time can both affect her cycles as well as how much attention she's paying to her own body - even before the revelation of the affair, I believe she was busy having taken on extra work due to loss of income, then the clinic and Nandini's arrival in their lives, then Kunal's weird behavior and finally the revelation. She's had a very busy few months, so it's quite possible her cycles are messed up and she knows it, but hasn't followed up because she assumed it's from the stress/removal of IUD. Now whether they test for pregnancy while removing the IUD, I don't know because, again there are many different ones and I haven't personally used one.
Also, alcohol consumption is wrong in general but really only can be damaging for a fetus if it's during certain months of the pregnancy and if it's a constant and/or large volumes of consumption. That's mainly coz alcohol can affect the development of the fetus in certain stages. But, not all men/women react similarly to alcohol and therefore, alcohol consumption during pregnancy is not similarly bad for all pregnant women. It's just easier to tell all pregnant women/women trying to conceive that they should watch their alcohol consumption since it's harder, more time consuming and more expensive to test every woman's tolerance (many women don't even have access to doctors, much less these tests).