This seems to be a frequently asked question all around this forum so I figured I'd chime in here at least.
The scientific link between alcohol and birth defects is fairly new. There have been casual connections made throughout history, most of which were viewed as more moral or hereditary issues. It wasn't until 1973 the the first scientific study linking alcohol with fetal defects was published and even then, it took more than a decade for it to really start trickling down to practicing doctors such that they would 'suggest' their patients merely lower their alcohol consumption rather than stop altogether. ffs, during the 70's and 80's, doctors were actually using alcohol in IVs to help stop preterm labor. Even when it became widely accepted that alcohol and fetuses don't mix, there was still a lot of disagreement about exactly how much alcohol a pregnant woman could drink before it became harmful. It wasn't until this year that the CDC and other groups came out with a very definitive 'no' on drinking even small amounts of alcohol when pregnant and even went so far as to recommend women not drink at all if they are sexually active and not on birth control.
Long story short, a 1940's woman really would have had no reason to think she was actively harming the fetus while consuming alcohol. And as has been mentioned, the sanitation and sewage concerns of mid 18th century Paris would present its own problems. However, a WWII nurse, especially one who had lived all over the world as a child, should be quite knowledgeable about many different water purification techniques. Though, considering iodine hadn't yet been discovered, bleach hadn't been invented, the methods are limited and more physically intensive. Though, since Claire wouldn't have known what we know about alcohol and fetuses, she'd really have had no reason to concern herself with it. Alcohol was probably in everything, including added to boiling water used to make tea, even if only to mask the nasty smell of the Parisian water.