OK, it's not a guess. I'll call it informed speculation. If I had to put money on it, I'd say that the mom T Rex was a flat-chested, egg-laying reptile. But there is nothing wrong with thinking outside the box on this matter either and there are enough missing pieces to allow for it.
You're working your way there, but you're still not showing that ability to take off your creationist hat and put on the evolutionist hat. I think it was jennyp who speculated that probably any of the usual E-side suspects could log on with a new name and convincingly portray a creationist. She doubted that any of the then-usual C-side players could do the mirror-image thing, pretend to be an evolutionist. (We might have to make an exception these days for AndrewC, data lawyer extraordinaire.)
Here's the problem for your oversimplification and a key to your answer. Evolution says that mammals arose from reptiles. Why, some of the dinos even appear to be growing warm-bloodedness to support their increasing levels of physical activity! (We can tell from microscopic analysis of the blood channels in their bones.)
And yet no one reasonably versed in the evolutionary story would happily accept the possibility of mammaries on Mrs. T-rex.