On her marriage day, she ceased to become Cissa and instead transformed into Narcissa.
She is pregnant. Barely. Her stomach is still smooth, her skin still taut with youth. The only marks of her condition is the conspicuous absence of her monthly bleeding. She almost misses it.
It was like a comforting friend, who visited every month. It might have caused problems and embarrassed her, but, like any houseguest, it soon enough left. Now, even it wasn't here, and Narcissa felt totally alone.
Lucius is always away. And it wasn't like she wished to be married.