Until recently, the UK guidelines were that one or two drinks, one or two times a week, was absolutely fine. The change in guidelines, AFAIK, was because people tend to kid themselves about how much they're drinking, and nobody wanted to be liable for that. There has never been any evidence that a small amount of alcohol was harmful, but on the other hand, nowadays we don't really have evidence to say that it's not. It's a change of emphasis.
Here, for example, are the [ www.nice.org.uk/guidance/cg62/chapter/1-guidance#lifestyle-considerations UK guidelines from 2008] :
1.3.9 Alcohol consumption in pregnancy
1.3.9.1 Pregnant women and women planning a pregnancy should be advised to avoid drinking alcohol in the first 3 months of pregnancy if possible because it may be associated with an increased risk of miscarriage. [2008]
1.3.9.2 If women choose to drink alcohol during pregnancy they should be advised to drink no more than 1 to 2 UK units once or twice a week (1 unit equals half a pint of ordinary strength lager or beer, or one shot [25 ml] of spirits. One small [125 ml] glass of wine is equal to 1.5 UK units). Although there is uncertainty regarding a safe level of alcohol consumption in pregnancy, at this low level there is no evidence of harm to the unborn baby. [2008]
1.3.9.3 Women should be informed that getting drunk or binge drinking during pregnancy (defined as more than 5 standard drinks or 7.5 UK units on a single occasion) may be harmful to the unborn baby. [2008]