If I had to estimate a time line between the baby shower in Marco Jr. and Meteora's attack on Mewni in Divide/Conquer, here's the basic setup.
On average, women have a baby shower 32 weeks into pregnancy. Let's assume Angie had her baby shower at that time and Marco Jr. is due in 8 weeks.
Skooled! and Booth Buddies are likely to take place on the same day. As Meteora rips Pony Head's horn off, Star gets a call at the end of BB likely from one of her sisters telling the incident.
Bam Ui Pati! and Tough Love are likely on the same day as well. Let's skip a day assuming that it took a while for Pony Head's sisters to get her out of bed. Star was helping out Pony Head, Moon and Eclipsa search for Meteora, and with Marco off-screen, he is likely doing more squire training (he likely had to attend an event, and made a quick trip visit to Earth. Wishful thinking).
So it would be about 3 days between Skooled! and Tough Love.
As Moon went missing after Eclipsa defended her from stopping Meteora, Star was acting queen in Divide and Conquer for 4 days in according to the Book of Spells. As Star gave the wand to Eclipsa, it leaves the show on a cliffhanger.
By my calculations, these 4 half hour episodes spanned in between 7 days.
So as of the end of Season 3, Angie is 7 weeks due.
But seeing in "Butterfly Follies" and "Escape from the Pie Folk" while looking for Moon, Eclipsa is already queen and outlawed imprisoning monsters, but the Mewmans are making fun of the Butterflies for their recent actions. So it would be about a week in between episodes (Conquer and Butterfly Follies).
Angie at the moment now would be six weeks due before the baby is born.
In conclusion, Marco hasn't been gone long enough after the baby shower but in those 2 weeks these series of events are flying at him. In other words, nothing to worry about for me.
It might be some wishful thinking, but I bet Marco will visit them just in time when the baby is born.