Membership in the Southern Baptist Convention has topped the 15 million mark, fueled in part by the largest increase in baptisms in a decade.

Statistics reported through Uniform Church Letters processed by the Southern Baptist Sunday School Board in Nashville show 385,031 baptisms reported for 1990, compared to 351,107 in 1989. The new figure represents a 9.7 percent increase that was attributed partly to a series of "Here's Hope" simultaneous revivals held throughout the country last year.

Church membership reached 15,044,413, registering a 0.9 percent gain of 130,875, the largest increase since 1986. The number of churches increased by 189 or 0.5 percent to 37,974, while total tithes, offerings and special gifts reached $4,567,834,980, a 6 percent increase of $258,496,000 over 1989.

The one national program that registered a decrease was the Woman's Missionary Union, which showed an enrollment decline of 0.4 percent, or 4,476, for a new total of 1,197,987. This was the second consecutive decrease for the missionary union enrollment, having dropped 0.1 percent the previous year.

Asked what factors might account for the decreases, Karen Benson, communications group manager for the women's auxiliary, said, "We don't know, but I can tell you that we're looking." She said the missionary union also has been experiencing a decline in its magazine subscriptions and "sluggish product sales" in recent years and has been conducting a major research project to determine reasons for the decline.

Benson said possible explanations might include an increase in the number of women in the work force, "which cuts down on the amount of disposable time they might have for any activities," and a failure to attract younger women to membership. She said missionary union units operate in about 27,000 of the SBC's congregations, which means about 10,000 churches do not have the units.