Dr. Gero was a genius. A mad genius, yes, but a genius nontheless. He made super powerful androids like 17 and 18. And then, he made his most powerful android, Cell. Cell really wasn't an android, but he was able to absord both 17 and 18 into his body, making Cell more powerfull than a super saiyan..almost. Cell was, of course, defeated by Gohan. Before he was defeated, Cell spat out Android 18. She was covered in slime, but she was still alive. 18 remembers what was it like to be eaten by Cell. It was awful, it was painful and it was blind luck that she got spat out. 17 had been fully digested by Cell, and it was only a matter of time before 18 had the same fate as 17. But, who says that Cell was the only creation of Dr. Gero's who could absord humans?
Now it's been 7 long years since Cell was defeated. During that time, 18 had married Krillin and had a baby with him. 18 was once human and has some human parts left over, including a vagina and a womb. This also includes her stomach. 18's stomach is her real stomach, but thanks to Dr. Gero, 18's stomach has some unusual propeties. First, 18 is able to swallow any sized human down her throat. Second, 18 has full control of her bodily fuctions now that she's an android. This means that she could swallow someone and not digest them if she choosed. 18 never told anyone about her swallowing talents, even her own husband was unaware that she could gulp him down. Not only 18 could swallow people with her mouth, but with her rear end and twat as well. 18 had asked Dr. Gero for these special talents, and Gero was only too happy to give 18 them.