The changes address issues that arose this year when Serena Williams, who returned to the action after giving birth to her daughter, was unseeded for some events and caused an uproar with a black, skin-tight bodysuit she wore at the French Open.
The new rules will allow players out of competition for 52 weeks or longer to use their Special Ranking in 12 tournaments.
A player returning from pregnancy will have a three-year period to use her Special Ranking, which will now begin at the birth of the child.
For players who would qualify for a seeded position in the draw, the updated rule will ensure they will not face a seed in the opening rounds whether returning from pregnancy or injury.
Williams' ranking had fallen to No.451 when she was not seeded at the French Open.
The American was, however, seeded 25th at Wimbledon despite being ranked outside the world's top 32 players.
Former world No.1 Victoria Azarenka, who returned to tennis in mid-2017 after the birth of her son, welcomed the rule change.
"Our players should feel comfortable and confident to take time away from the courts to have a family or recover from injury and I think these new rules support that," Azarenka, who is on the a WTA Players' Council, said in a statement.
The board of directors also agreed that players at WTA tournaments would not be prohibited from wearing leggings or compression shorts without a skirt, dress or shorts over them.
The WTA had no rule explicitly banning a player from wearing such outfits but adjusted the language of its rule to be more clear.
Among other rule changes is the introduction of a 25-second shot clock between points at premier events, with a full rollout for all WTA tournaments in 2020, in a bid to speed up the pace of play.
Players will also only be allowed one toilet/change of attire break per match, down from two.