Ruth has gone to see an abortionist instead of a doctor. She is considering getting an abortion because she feels so desperate - she and Walter have been fighting a lot, the apartment in which they live is already really crowded, and she believes she needs to continue working to help support the family. So, she puts down a $5 down payment and makes plans to return for the abortion at a later date.
Ruth thinks, as some level, that this is the best solution given the circumstances, but if the reader looks into her heart, he or she sees that what Ruth really wants is for her relationship with Walter to get better and to have a close-knit family. Once Mama, or Lena, goes out and buys the house, Ruth gets excited thinking about all the possibilities - including having this new baby.
Keep in mind that this play is set in the late 1950's and that abortion at this time was illegal. Ruth cannot, therefore, go to a regular doctor. What she is seeking is what we now call a "back alley abortion," and often these resulted in serious health complications or even death for the woman because of untrained "doctors," unsanitary equiment, and lack of follow-up.
Mama picks up on what Ruth has done when Ruth slips and refers to the doctor she saw as "she" - Mama realizes at this point that Ruth did not see their regular doctor, and she later urges Walter to tell Ruth to keep this baby. She reminds them they've already given up one child to poverty - a child Mama had who died - and that they should not give up another.