Pregnant and 18 Month Old Is Acting like She Wants to Nurse.

Could be a mix if issues...

1) She probably was not ready to wean when you chose to do it
2) Separation Anxiety is still an issue at this age
3) She sees Mommy changing and you may not be giving her the same a mount of physical attention you did before your body started getting larger.

Discouraging her from nursing is like refusing your child a hug or kiss... let her nurse and chances are she will stop immediately for very shortly afterwards. It's more of "Will Mommy love me enough to do so" kind of thing.

I had the same experience! my kids are spaced about the same as yours. The funny thing is that I had a very hard time nursing my first child and didn't get the hang of down till she was 7 months old, and I weaned her at about a year. So it really surprised me when I was pregnant with #2 how interested in my breasts she was. When ever she had a chance her hand was down my shirt. lol.
We have always co-slept, but my oldest defiantly became more clingy.
They know something is going on and it makes them a little insecure.
I do think hormones has a role in the interest with the breasts.

My two year old has been acting that way recently (somewhat). She knows a new baby is coming and there are days she has wanted to "play baby" all day. She imitates babies and likes to cuddle and pretend she is using a bottle and wants a lot of reassurance. On the other hand, she still likes to be the "big girl" and be independent.

With the breast thing, I would just gently push her away and giggle and say "that is silly!" pulling up your shirt may just be out of curiosity, but a simple "No please!" may work. My daughter is interested in breastfeeding again, but I tell her she can play "baby" with her dolls. She weaned at 21.5 months and is almost 28 months now. We are due in about 2 months.

Just be re-assuring. Be excited about the new baby, but show her that she is still really important. When we get a lot of stuff for the new baby, we try to get her something too (not big... but if we are buying baby clothes, we get a t-shirt or something that she gets to pick herself) The point is that even at 18 months she probably has caught on to the idea that someone is threatening her "position" in the family.... and you just need to help her understand that she will still be just as loved, and just as important to you as when it was "just her".

After the baby comes... well... talk to other moms about that! I still am crossing my fingers hoping that adventure won't be too rocky!

Good Luck and Congratulations to your family!
-M.