Why in this advise :
The risk of harm to the baby is likely to be low if a woman has drunk only small amounts of alcohol before she knew she was pregnant or during pregnancy
They use "has drunk", but not "had drunk".
The medical writer is giving general advice, meant to apply to any pregnant mother, past or future. It's similar to how English uses the simple present when offering instructions:
To start the car, first turn the key
The car is likely to start if you have kept it in good condition.
We know that this meant to be general advice because the writer says "a woman" and not "the woman"* or "Mrs. Baker".
On top of this, the context indicates that this is a current case, because of the use of the present tense:
the risk to the baby is likely to be low
If this was a past case, the writer would have instead said:
the risk to the baby was likely to have been low.
With a current case, the mother may continue to drink alcohol in the future, so it's an ongoing condition, and the present perfect is appropriate. However, if the writer wants to make it clear that the mother definitely will not drink again while pregnant, the past perfect might be appropriate:
The risk to the baby is likely to be low because Mrs. Baker had drunk only small amounts before she knew she was pregnant.
Again, this would have to be a known person, otherwise the writer could not say for certain that she had stopped drinking.