LONDON (AP) — A new study shows that women in Britain earn consistently less once they have children — even though the overall gender pay gap has narrowed over the past two decades.

The study by the Institute of Fiscal Studies said Tuesday that women find that by the time their first child reaches the age of 12, their hourly pay is down 33 percent versus that of men.

The report says that the difference could be attributed to women putting in fewer hours at the office and missing out on promotions as a result.

Overall, women in paid work receive 18 percent less per hour than men, compared to 28 percent less in 1993. The closing of the overall gap was down to improvements in the pay rates of less well-qualified women.