The United States of America Mathematical Olympiad (USAMO) is an invitation only high school mathematics olympiad that is held annually in the United States
MAA (Mathematical Association of America) was kind enough to provide me with various statistics of USA(J)MOs for the past five years. Unfortunately, given the drastic drop in number of female students who qualify for the AIME, the trend of female qualifications in USA(J)MO is equally discouraging. As you see below, the qualification of girls have stayed mostly constant over the past five years. Each year nearly 500 students qualify for USA(J)MO and the average number of girls who qualify has stayed constant ~10%.
I also found it interesting that the average scored of males vs females were not that different, however, most of the high scoring students in USA(J)MO are males. MAA now invites a few girls based on their scores in USA(J)MO to Mathematical Olympiad Summer Program (MOSP) who are then further trained for European Girls Mathematical Olympiad.