Certain types of shell fish are not healthy during pregnancy but eating clam chowder is perfectly safe and fine during pregnancy.
I ate clam chowder every few days or so during my pregnancy and my baby boy turned out very healthy and I had no problems during the pregnancy or birth of my baby.
During pregnancy you can safely eat clam chowder as well as seared flounder filet without any problems and you'll likely have a healthy pregnancy.
So go ahead and enjoy your clam chowder while pregnant because it's perfectly okay.