The U.K.’s Gender Pay Gap Skyrockets After Women Have Children by Claire Zillman @FortuneMagazine August 23, 2016, 10:12 AM EDT E-mail Tweet Facebook Linkedin Share icons In her first speech as prime minister in July, Theresa May cited Britain’s wage gap—”If you’re a woman you will earn less than a man”—as one of the “burning” injustices that she planned to fight against. A new report out Tuesday illustrates just how tough a battle that will be. The Institute for Fiscal Studies’ research found that women in the U.K. earn 18% less than men on average. That’s down from 23% in 2003 and 28% in 1993, but the wage gap remains stubborn, especially for the better educated. The hourly pay of higher-educated men and women has not shrunk at all in the last two decades. The declining pay gap is instead due to more women becoming highly educated and the reduction in the pay gap among the lowest-educated individuals. (For comparison, in its database of countries’ gender wage gaps, the OECD pegs the United States’ at 17.91% and the U.K.’s at 17.48%, just below the IFS’s figure. New Zealand leads all OECD countries with a gap of just 5.62%.) Tuesday’s IFS report also indicates that the gender wage gap in the U.K. balloons as workers enter their late 20s and early 30s—men’s wages continue to grow rapidly, while women’s plateau. That change is tied quite clearly to motherhood. The report says there is a gradual but continual rise in the wage gap after the arrival of a woman’s first child. By the time that kid is 12 years old, a woman’s hourly wages are a third below men’s. If women leave paid work and then return, their hourly wages are 2% lower for every year they were not employed. That correlation is even worse—a 4% reduction—for women who have at least A-level qualifications, which U.K. students receive when they complete their secondary education. Interestingly enough, that relationship does not exist for lower-educated women, since they have less wage progression to miss out on when they take time off. The IFS report is the second piece of bad news for working moms in the U.K. this week. On Monday, British charity Citizens Advice released a report concluding that women in the U.K. are increasingly concerned with workplace discrimination as it relates to maternity leave. The nonprofit recorded a 58% jump in the past two years in the number of women coming in for in-person consultations on the issue. Theresa May’s vow to close the pay gap continues former prime minister David Cameron’s promise to end the disparity within a generation. The U.K. government has introduced new rules aimed at shrinking it, such as forcing companies with at least 250 employees to publish their pay gaps starting next year. It’s also providing more free childcare and shared parental leave, but there’s evidence that few fathers are taking it.