A wave of successful initial public offerings is propelling First Trust US IPO Index to hefty gains.

The exchange-traded fund, which tracks an index of 100 U.S. companies that went public or were spun off in big, well-received transactions in recent years, gained 31.1% in the 12 months through March, outpacing the S&P 500's 21.9% gain and the average 25.5% gain of funds in its large-growth category, according to Morningstar Inc.