-니까 is an emphasizing form of -니 -니 is used for reason, cause or precondition of the following clause. by contrast, -어서 is used for reason, cause of the following clause. When it used for cause 임산부도 뱃속에 아기가 있으니까 몸이 무겁잖아요. can be replaced with 임산부도 뱃속에 아기가 있어서 몸이 무겁잖아요. When it used for precondition 배가 고프니까 밥을 같이 먹을래요? cannot be replaced with 배가 고파서 밥을 같이 먹을래요?(X)

It is conjunctive. It means 'as' or 'so' other ex) 밥을 먹다 so 배가 부르다. → 밥을 먹으니까 배가 부르다.

-니까 here is a conjunctive suffix used to give a reason or cause to the following clause. 뱃속에 아기가 있으니까 there are babies inside their stomach(⇒reason/cause) 몸이 무겁잖아요 their bodies are heavy. So the sentence reads like 'Because pregnant women have babies inside their stomach, their bodies are heavy'.