One out of every three babies in Spain is born to unwed parents, twice as many as 10 years ago. The decline in marriages, the rise in single mothers, immigration and a more secular society have all contributed to this trend, which brings Spain more in line with the European Union average.

Since 1981, when the law eliminated differences between children born in and out of wedlock, the proportion of the latter has risen steadily. If it was 4.4 percent that year, by 2000 it was 17.7 percent, and in 2009 — the last year for which the statistics office holds data — that rate had grown to 34.5 percent, or 170,604 babies.

"These are astounding figures," said Constanza Tobío, a professor of sociology at Madrid's Carlos III University. "Couples have become more modern, and Spanish mothers have quickly become almost like Swedish or British mothers on that front. This change is the result of the secularization of society, of tolerance, of mothers' autonomy — they no longer need the safety of marriage to procreate — and of legal equality for children regardless of their parents' civil status."