As for suggestions that Millie hasn't announced because she's handing the baby to Sue, that's utterly ridiculous. It totally goes against the leaked text for starters, where Millie refers to HER son.
I'm pretty sure she hasn't announced to avoid the backlash that 2 babies 18 months apart, under the age of 21, by 2 different fathers will bring. And that's her prerogative not to announce.
So many people choose not to announce their pregnancies on social media. I have several acquaintances (old uni friends, old school friends, old work colleagues etc) who haven't announced their pregnancies on FB etc, and the first those of us who don't see them day to day in 'real life' now know about the baby is when they post the birth announcement. It's really not that unusual, pregnancy can be a really private thing to some.
It's only seen as strange because Millie is a z list social media 'star', due to her family being famous for shagging, and because of the fuss they made last time (they, not necessarily Millie who might not have actually wanted all that fuss). If she was just someone you worked with, for example, would you really think it's odd if you notice she's never announced her pregnancy on social media? I'm pretty sure she won't be hiding it in real life (as it'll be pretty impossible by now anyway!). Not announcing on social media isn't really the same as 'hiding' a pregnancy. She's certainly posted comments that people have seen that allude to a pregnancy. Her true friends will all be well aware, even if her social media wannabe friends won't believe it until they see her announce.
Her pregnancy, her choice whether to announce on social media or not. She's got this far without doing it so I'm fairly sure she won't now.