The CDC made this recommendation in 2016 and there was quite a furore about it then too.
But as with so many of these attempts to change our behaviour, there is no research providing robust evidence that moderate to low amounts of alcohol cause FAS. A number of literature reviews of the available research confirmed this to be the case, including one review from 2006 carried out by Oxford University researchers.
There is ample evidence that about 5% or so of babies whose mothers abuse alcohol in pregnancy will be born suffering from fetal alcohol syndrome, which is serious and causes lifelong issues for the child.
There is also ample evidence that the reason why alcohol abuse doesn't always cause FAS has to do with the overall situation of the mother. In many cases where FAS is present in the child, the mother often has a number of other issues such as drug abuse, malnutrition, domestic abuse and so on. (FAOD, FAS can and does still happen just due to alcohol abuse.)
There is very limited data on the effect of moderate to low amounts of alcohol intake on the fetus and what studies there are, are frequently suffering from methodology issues.
Nonetheless women are expected to forego even a single drop of alcohol even in pre-pregnancy (IIRC, that's what the CDC called the status of non-pregnant women of childbearing age who might get pregnant, including those who aren't even TTC.)
Because if you don't, and the baby has the slightest issue, it's all your fault.
There was a recent thread on AIBU where a woman who didn't drink at all in the first trimester and now has one glass of wine a week, was receiving some horrendously accusatory comments, going way beyond a robust challenge of the OP's considered belief that one glass a week was not a risk to the baby. It did not matter that the OP had researched the issue and then self-regulated her intake to one small glass of wine a week after talking to her HCP. And for most weeks she stuck to that amount. (she'd had three weeks with no alcohol and three weeks with two glasses, and the rest were one glass). Nonetheless, the OP was accused of being an alcoholic and someone who didn't deserve to have a baby, because if the unborn baby didn't come first in every respect, she was unfit to be a mother.
All of this despite the fact that the dos and don'ts of pregnancy are rarely static. When I was first pregnant, there was real panic about the effect of sweeteners. Now they're considered just fine. Feeling nauseous 24/7, I didn't drink while pregnant or nursing, but back then the advice was to drink in moderation, but drinking was fine. Around about 2006, when I was last pregnant, I remember is when all the advice came in that said no alcohol whatsoever in pregnancy (but not to worry if you didn't know you were pregnant and drank) and in 2016 the CDC said no alcohol whatsoever in pre-pregnant women either.
Both recommendations were made without the publication of a solid evidence base accompanying the announcements. I'm very curious to see whether the WHO proposal now is accompanied by new research that does stand up to scrutiny for a change.
(Obviously, a lack of evidence for harm is not evidence of an absence for harm, so I remain open-minded about future developments providing robust evidence that low-level alcohol intake leads to FAS. But there is none right now.)