Eating seafood is generally not advised if it has high mercury content for pregnant ladies. However my ObGyn had told me to take care but if I really wanted to have something, I could have it provided I choose my meals wisely. I second Aparna, didn't have prawns for the first trimester, after that I am relishing seafood as and when I want to. However, please avoid seafood with high mercury content.

I'm not an expert but I'm a seafood lover and have relished prawns all my life. However my ObGyn advised me to be careful for first 3 months so I didn't have any. After that I had a lot. So unless your wife has any aversion, nausea or allergy towards prawns or shelled seafood, I don't see why she shouldn't have them.