Weight loss, when do we gain?

For those of you who’ve lost weight so far through your pregnancy, when did you start to gain it back? Or are you still losing?

I’ve lost a little over 15 pounds since being pregnant, and while it was a nice little surprise (I was overweight when I got pregnant) I want to make sure I’m gaining where I should for the sake of the baby.

I’m 18 weeks 1 day, and for the last week or so I’ve been holding steady at about a pound above the lowest I’ve been (which was around 13 or 14 weeks, went back up around 16 weeks now I’m back down).

Should I be gaining weight yet? Or when should I be concerned if I haven’t gained?

Thank you for any advice you can offer!