You need to up your calorie intake as you're eating for 3 now! That's probably one of the ways.
However milk supply dip during pregnancy is usually caused by a change in hormones and you may be comforted to know that only a minority of mums do not experience a decrease in milk supply.
Your child may decide to self-wean when your milk production decreases accompanied by the change in taste of the milk due to increased colostrum in your milk also. Be prepared to make up for it by increasing solid intake.
http://kellymom.com/tandem-faq/16milkchanges/
I believe it is still possible to nurse as per usual and just pay more attention to your back support.
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