Hermione would never announce her engagement to Ron at a dinner party that Ginny threw--even if it wasn't Ginny's big announcement night. Now that Ginny had her big news, Hermione was thinking that she and Ron would wait a few weeks until things settled to talk about their upcoming nuptials and announce their engagement. Besides, people might think it a little less strange if Ron and Hermione claimed to become engaged after moving in together.
"So long as you reach the same destination, the journey itself doesn't always matter." Hermione agreed, grinning as she ran her hands up and down Ginny's upper arms once in a show of support. "When's the baby due?" She asked, letting go of Ginny to slip her hands into her pockets--her left hand fingering the ring in her pocket gently.