China's minimum paid marriage leave is three days, but provinces have been able to set their own more generous allowances since February.
The northwestern province of Gansu and the coal-producing province of Shanxi now give 30 days, while Shanghai gives 10 and Sichuan still only three, according to the People's Daily Health.
"Extending marriage leave is one of the effective ways of increasing the fertility rate," Yang Haiyang, dean of the Social Development Research Institute of Southwestern University of Finance and Economics, was quoted as saying.
"The extension of marriage leave is mainly in some provinces and cities with relatively slow economic development," he said, adding that there was an urgent need to both expand the labor force and stimulate consumption.
Yang said a host of other supporting policies were still needed, including housing subsidies and paid paternity leave for men.
China's population fell last year for the first time in six decades, according to official data - a turning-point that is expected to mark the start of a long period of decline.
Last year, China recorded its lowest ever birth rate, of 6.77 births per 1,000 people.
Much of the downturn is the result of a "one child" policy imposed between 1980 and 2015, and a surge in education costs that has put many Chinese off having more than one child, or even having any at all.