Image copyright PA Image caption The public health guidance said women might be exposing their developing baby to alcohol without knowing it

Public health communication can be a tricky art. The science needs to be sound, of course, but so does the tone.

The US-based Centres for Disease and Control (CDC)'s latest advice, on drinking during pregnancy, has been ridiculed online for broadening the warning out to millions of American women who are not pregnant.

Scientists differ on how much alcohol is safe to drink in pregnancy, so health authorities tend to recommend not having any at all. The CDC took the same approach, which could be called "better safe than sorry".

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But their guidance went a step further, to say that sexually-active women of childbearing age should stop drinking if they were not using contraception. This was in case they got pregnant accidentally.

"Why take the risk?" it said, adding that "more than three million US women are at risk of exposing their developing baby to alcohol because they are drinking, having sex, and not using birth control to prevent pregnancy".

And that is what the online outcry focused on.

Some people said it asked women to think too much about hypothetical babies, and that women who did not want to have children and had no intention of becoming pregnant were being made to asked to change their behaviour just in case.

"Critics may argue that there's little sacrifice in avoiding sex or drinking while not using contraception, but invoking the health of a baby that doesn't yet exist radically changes that equation", Rebecca Ruiz argued in Mashable.

Image copyright @foyobil

Others felt it reduced women to the ability of their wombs to carry babies.

Image copyright @Lyndsay_Kirkham

"The language insinuates that your womb is a Schrodinger's box and you shouldn't pour alcohol into it unless you've peeked in there to be 100 percent sure the coast is clear", said one article on the The Atlantic.

Critics suggested that the tone had "shamed" women for drinking.

One Twitter user made a parody of a chart used by the CDC, modifying it to warn women that drinking too much could lead to online shopping while under the influence.

Image copyright @TheSuperSous

But some came to the CDC's defence.

Writing in the Huffington Post, Erin Schumaker said the CDC were only trying to warn women who were trying to get pregnant. She admitted the agency "could use some help on press releases".

The CDC said:

"About half of all US pregnancies are unplanned and, even if planned, most women do not know they are pregnant until they are 4-6 weeks into the pregnancy.

"This means a woman might be drinking and exposing her developing baby to alcohol without knowing it."

The CDC's advice on pregnancy and alcohol states known risks, agreed on by experts, to the mother and baby.

The risks of drinking during pregnancy include miscarriage, stillbirth and foetal alcohol syndrome.

This ties in with advice given in the UK, where the Chief Medical Officers advise women who are pregnant, or planning to become pregnant that it is safest not to drink at all in order to minimise the risks to the baby.

The Royal College of Obstetricians and Gynaecologists in the UK has also said that the period around conception and the first three months of pregnancy are the most risky.

By way of comparison, Australian health authorities warn mums-to-be against getting drunk, but say: "If you drank small amounts of alcohol before you knew you were pregnant, be reassured that the risk of harm to your baby is low."

Experts also agree men should look at their alcohol intake if they want to conceive with their partner. Heavy drinking can affect sperm quality.