More women in the north of England smoke while pregnant compared with the south, official figures for the last three months of 2011 have shown.
The data, published by the NHS Information Centre covering 167,300 pregnancies, showed 20% of women in the North East were smokers when they gave birth, compared with 6% in London.
The overall figure for England is 13.4%, down from 15% in 2006-07.
Smoking in pregnancy is linked to low birthweight and premature delivery.
Of the pregnant women in England who classed themselves as smokers at the time of giving birth, rates varied from 2.8% in Brent, London, to 30.3% in Blackpool.
Smoking rates were higher in every northern strategic health authority, compared with those in the south.
The data was provided by GP practices via 149 primary care trusts.
Previous data has shown that, of mothers who smoked before or during their pregnancies, just over half gave up at some point before the birth.
Babies from deprived backgrounds are more likely to be born to mothers who smoke and to have a much greater exposure to secondhand smoke during childhood.
Cutting smoking during pregnancy is one of three national aims in the government's tobacco control plan, published in March last year.
The aim is to have cut the number of women smoking at the time of giving birth to no more than 11% by the end of 2015.
NHS Information Centre chief executive Tim Straughan said: "Smoking can cause a range of serious health problems, including lower birthweight, pre-term birth, placental complications and perinatal mortality.
"The statistics we have published today highlight stark regional variation in the proportion of women smoking at the time of giving birth.
"They will be of considerable interest to those responsible for promoting good health during and after pregnancy."
Public Health Minister Anne Milton said: "The regional variation we are seeing here is unacceptable and we want to reduce rates of smoking throughout pregnancy across all areas of the country.
"NHS doctors, nurses and midwives are working hard with their local communities to tackle this. We are also giving councils the power and the budget to tackle issues like this locally."
Data from a 2010 UK-wide report showed 16% of women in Wales had smoked throughout pregnancy, compared with 13% in Scotland and 15% in Northern Ireland.