The first time she isn't present with them is she has been told to go the the drug-store, this leaves Blanche and Stanley to grow their relationship. For example when she returns, Blanche openly admits that they wee flirting as she says "I was flirting with your husband Stella!". Whilst also Stella not saying anything. So this could help us understand more of Stella as a character, she is subserviant and doesn't question anything, portraying her passiveness. The second time she is absent is when she goes into labour, and in this scene, Stanley rapes Blanche. This could show the transition of their realtionship beginnin with a playful natue of Blanche sayin "I was fishing fo a compliment, Stanley" to him raping her.