Lucifer cannot get pregnant again unless he uses an ancient blood spell, or if he's lain with another angel. Charlie was a very special miracle, and while Lucifer likes the idea of giving her siblings someday, he knows it's never going to happen. Possibly why he overcompensates when he's with her.
(feel free to disregard if you're not into the "Angels Lay Eggs" verses)
Due to angels being fairly immortal, angelic heats don't hit very often. Maybe every fifty years or so. Their hormones do prepare them to lay an egg, whether it's fertilized or not; if the egg is not fertilized, then their hormones react as if they've had a miscarriage, and they'll have an emotional drop for several days up to a week.
When Lucifer goes into heat, his wings are out, all the time. His feathers are brighter than usual, as if freshly washed, and they have a faintly sweet scent that emits from the oil glands hidden away within the feathers. He's constantly preening or showing them off.
He will get very flirty. Extremely flirty. Like "he's had a few stiff drinks and his inhibitions are lowered" flirty. He would even flirt with Alastor, despite knowing the outcomes (unless any Alastors like Radio/Apple, pspspsps)
He will sing more than he usually does, often love songs or sensual songs.
When his body is preparing the egg laying, he gets nesty, choosing a room to make an actual nest in. He can and will steal people's clothing items for their scents. Your favorite shirt? His now. Your pillows? His now. Any stuffed animals? His now. You're not getting those back for a while.
He will drink so much milk. So much. He's seen with a glass of milk near him at all times. One time Charlie walked in on him guzzling a full gallon of it.
He will gain some weight. It will disappear after the egg is laid.
A fertilized egg will change colors; an unfertilized egg will be a plain white. Charlie's egg was a beautiful lavender and gold.
Lucifer's heat will last for a whole month, as he only lays the egg in the last week to give it time to be fertilized. It takes another week, to 10 days, before the egg itself will hatch.
Because my Lucifer carried and gave birth to Charlie, I imagine he and Lilith shut themselves away in the Morningstar palace for the duration of the pregnancy. The announcement would be made that they were expecting a child but not who was carrying it, and Sinners and the other demons would just assume they're not risking yet another miscarriage.
And if -- in a verse where this was possible -- Charlie hatched from an angelic egg, Lucifer was broody as hell, never leaving the egg for a single second. They made absolutely sure that she would survive.
I know now it's a fake screenshot, but I do want to incorporate it into my Lucifer's background that he gave birth to Charlie due to the myths of Lilith being unable to bear a living child from her womb. They tried for centuries, but after many miscarriages, Lilith wanted to give up, and Lucifer was able to use blood magic as a loophole. He's now capable of pregnancy, but only if he allows it; this is a closely guarded secret between himself and Lilith. Not even Charlie knows.
Of course, if anyone does not want to go down this route in threads and backstories, I'm more than happy to disregard it. But for my Lucifer specifically, he did give birth to Charlie, because he wanted to make Lilith happy, and they both adore their baby girl.