For the first time ever, the U.S. Census Bureau now counts married same-sex couples in its survey of married Americans, and despite a 35 percent increase from last year, same-sex couples make up less than one half of 1 percent of all married couples in the U.S.

There are an estimated 252,000 married same-sex couples in the United States, out of 56 million total married couples. That’s up from 182,000 in 2012.

The Census has acknowledged it’s had a hard time reaching an accurate number. A “significant” part of the increase is due to a change in how it uses incomplete questionnaires. For the latest numbers, when forms didn’t include information about respondents’ gender, a gender was assigned by the Census Bureau based on first name, using an index that showed the likelihood of someone with a certain first name being male or female (for example, in Wyoming, someone named Jean is more likely to be female, while in Louisiana, they’re more likely to be male).

How many married same-sex couples in 2013 vs. 2012:

The Census Bureau has called its struggle to reach an accurate number of same-sex couples a “very serious problem.” It stems from forms that do not directly ask about same-sex marriage, instead using estimates based on gender and questions about the relationships between people living in the same house.

The survey also looked at how many same-sex households in each state are married. The responses were self reported, so it includes couples who are legally married, but also couples that consider themselves married even if they were not, or even if they live in a state where it isn’t legal.

Percentage of same-sex households where respondent identifies as a same-sex spouse:

Marriage for same-sex couples is legal in 19 states and the District of Columbia. Some states where it’s legal had a high percentage of married gay couples — Massachusetts and New Hampshire are at 59 percent and 57 percent, respectively. But in South Dakota and Wyoming, where it isn’t legal, more than half of same-sex households say they are married.