Apparently it's OK for pregnant women to smoke.

From The Metro:

Passive smoke causes permanent genetic mutations in newborns that is 'very similar' to the damage caused by mothers who enjoy a cigarette, according to experts...

Experts identified a smoke-induced mutation in GPA or glycophorin A – an 'oncogene' which changes normal cells into cancerous tumours. The GPA mutation was the same level and type in newborns of mothers who were active smokers and of non-smoking mothers exposed to tobacco smoke, said Dr Stephen Grant, head researcher from the University of Pittsburgh.

If this were true (and according to Dearieme's rule of thumb, it isn't), then unless a pregnant woman can avoid 'passive smoke' (that's a new one on me - presumably 'passive smoke' is what 'passive smokers' inhale?) absolutely and completely (which is probably impossible) then she may as well light up herself (should she so wish).

Nanny takes a different view, of course:

Pregnant women should not only stop smoking, but be aware of their exposure to tobacco smoke from other family members, work and social situations,' he added.

This practice is also known as 'generating a climate of fear in order to recruit an ever larger army of unpaid bansturbators'.