[My top 4 theories on who Zero's mother is.]
(1). Anne Yuri is Zero's mother and it is possible for Ultras to have children with Humans after all Ken Sato is the son of an Ultra and a human woman, and Ultraman Nice has two kids with a human woman. (Though it's still debated if they're actually his bio kids or step-kids. I like to think they're his bio kids.) So the theory isn't too far fetch'd.
(2). Zero's mother died when Zero was still a baby which why she's never mentioned by anyone.
(3). Zero wasn't born in a gestation pod but was concieved/born naturally and was an whoops baby, Seven and mystery mom tried to make it work but it didn't work and went their separate ways.
(4). Zero is a donor baby, mom was never in picture 'cos she was someone who donated her eggs/DNA to some kind of Gestation program. Seven never actually met her; she was just one of the many profiles Seven given to look over and he chose her DNA, popped that into a gestation pod and made Zero.
