It takes approximately 2 weeks for deconditioning to begin. Try to keep a maximum of 14 days break during busy/stressful times when your workouts fall to the wayside; otherwise, you'll be working to recover strengths that you have already worked hard to achieve! Fortunately you have built "muscle memory" so recovery is faster than initial conditioning.
Prenatally it is ESPECIALLY important to maintain conditioning with consistency. Without consistency, exercise prenatally becomes a risk.