Are you actually serious? Not joking?

No, I'm not joking. Evidently I have a very different take on this subject than you do.

We don't know if a pregnant woman drinking a couple of units a week is doing any harm to her child. Thus, the advice is to avoid all alcohol.

Yes; we don't know if it will cause harm, and it's avoidable--at least, that's the assumption underlying the advice.

It makes people ignore much clearer health advice. (This thread has someone suggesting that the drinking advice is similar to the smoking advice, as an example.)

So you're saying that, if we tell a person that drinking might harm her child, and smoking might harm her child, she'll ignore the smoking advice because she doesn't agree with the drinking advice; but if we don't tell her that the drinking might be harmful, she'll be more likely to listen when we tell her that the smoking is harmful? (I haven't seen the subthread you're referring to, so I'm not sure exactly what the argument is.)

I see several problems with this argument. First, many (I would say most) people can drink in moderation without becoming addicted, and can stop drinking, if given a good reason, without suffering significant harm. That's not true of smoking; I'm not aware of any significant number of people who can smoke in moderation without becoming addicted, or who can stop, if they choose, without suffering significant harm in the short term. If a person is a regular smoker, quitting smoking is going to put them under severe stress. Obviously that changes the risk-benefit calculation, but the fact that such a calculation might apply to an addict doesn't mean it applies to a non-addict.

Second, given the overwhelming evidence about the health risks of smoking, a person who has chosen to smoke has already decided to ignore a lot of potential risk. I don't think such a person will be easily convinced to change that behavior if they become pregnant (and, as I said above, it will be very hard and stressful for them to change it even if they try to).

Third, you're basically saying we should hide facts from people because we don't think they will make wise use of the information. I'll comment on that further below.

It also causes a great deal of stress to women. Some women may drink their regular amounts and then be very distressed when they discover that they are pregnant and have been drinking during the pregnancy. For most women the stress they experience is more harmful than the alcohol.

I'm not sure I understand this argument. As you're presenting the situation, it doesn't seem like the choice is between drinking and less stress, vs. not drinking and more stress. The drinking is the cause of the stress: the woman is worried that she might be harming her child. The rational response to that is for her to stop drinking, not to try to convince herself that the drinking doesn't really incur any risk to her child so she won't stress about it.

As for discovering you're pregnant and worrying about the fact that you've been drinking, if you're that worried about the risk of drinking to your child, why wouldn't you stop drinking if you are engaging in activities that might get you pregnant, just to be safe?

You assume that we know what the risks are, and that we can communicate those risks to pregnant women, and that they have power to control the risks they're exposed to.

We do know that alcohol poses some risk to a developing fetus. That's all I was assuming.

I don't understand the problem with communicating the risk; there are plenty of links in this thread to sources that give information about it, so the information that alcohol poses some risk to a developing fetus is out there.

As for women having the power to control the risks they're exposed to, sure, nobody can control all the risks they're exposed to, but that's why I specified "avoidable" risks in the post you responded to. Consumption of alcohol strikes me as a risk that is quite controllable and avoidable.

Women undergo a bunch of prenatal checks. To get to those checks they have to use some form of transport. Are the risks of travel outweighed by the benefits of the prenatal checks?

It depends on the relative risk vs. benefit. See further comments below.

There's no point in the 3d / 4d scans, and there's a possible risk of harm, so should the scans be banned? Or should we tell women not to get them?

No, and no. We should make sure that accurate information about the risks and benefits is available, and let women make their own decisions.

And who should we be criticising, the firms making money by pushing a possibly harmful but pointless scan to vulnerable women, or the women who get these scans?

If the scans really are pointless but carry a risk of harm, then the firm deserves criticism. If the women decide to get a scan that they know is pointless (or that the best evidence available to them says is pointless) but carries a risk of harm, then they deserve criticism.

It seems to me that we have a fundamentally different view of how this process is supposed to work. You seem to be assuming that there is some central authority that needs to decide what information to release and to whom. I see this as a bunch of individual actors, each of whom has a responsibility to provide accurate information and to make reasonable decisions based on the information they have.