Byleth x Dimitri - Rodrigue Jeralt Blaiddyd + daughters

After the war, there was an explosion of twins among Dimitri's friends. Byleth suspected it was some sort of blessing from Sothis either specifically to produce twins or just increased fertility. Byleth herself was not affected but she assumed it was because her body isn't normal and perhaps her ability to have any children was due to Sothis's blessing. She does go on to have two daughters in addition to her son with five years between each kid.

Claude x Petra - Tiana & Khalid

They have twins Tiana and Khalid and another son who has a large age gap from his older siblings.

Tiana gave Rodrigue a lion cub when they were five. Rodrigue proposed to her on the spot. Everybody thought it was cute but assumed they would get over their puppy love soon... they did not!

Tiana and Rodrigue's Official Engagement Party

Felix x Annette

Their eldest daughter is Glenn, second daughter Mercy and finally a son Alexi. They didn't have any twins but it is assumed that Annette's small frame couldn't have carried twins safely.

Ingrid x Sylvain

They were the first in their group of friends to have kids because they celebrated a bit too much together after the war. So Hugo and Alex were "oops" babies. Ingrid opted not to have any more kids for a long time because she wanted to work as a knight. She retired after over a decade and promptly had twin daughters Violet and Demeter.

Dedue x Flayn

Ethniu was born after the blessing of Sothis was assumed to have ended given the sharp decline in twins. But they also adopted Iris who is a Homunculus made from the blood, flesh and bones of Edelgard, whose remains were stolen by Agatheans. Iris was rescued from one of the last outposts of the Imperial resistance who were working with what remained of Those Who Slither in the Dark to destabilize and topple the United Kingdom of Fodlan. Ethniu is a dragon shifter and Iris has an anti-crest crest that was the result of TWSITD experimentation.

Ashe x Mercedes

Emily and Christopher

Alternative Ashe x Hapi

Christopher and Didi

Lorenz x Marianne

Edmund and Mary

Leonie x Raphael

Jeralt and also no twins as Leonie started having kids after the goddess’s blessing expired

Hilda x Seteth

Grimwald and Tuatha

Dorothea x Ferdinand

Multiple children but the eldest is Eddie which is not short for anything but rather a homage to Edelgard since that's what Dorothea used to call her. Eddie “consumed” his twin in the womb and was born with his twin's legs sticking out of his chest, which were surgically removed by Linhardt and so he doesn't have any deformities save for a scar but is still frequently bullied as a “monster” by everybody except Rodrigue and Grimwald. Rodrigue is his milk brother as Byleth feel in a deep slumber for six months after giving birth and Dorothea ended up breastfeeding both Eddie and Rodrigue.

Lysithea x Linhardt

Dawn and Caspian

Casper x Bernedette

Rudy, Ruby, Rosey, Jett, Sage, Coal, Pinky, Hazel, Lavender

Alternative Linhardt x Casper

Olinda a Homunculus created by Linhardt using the same technology as Iris. Older than Iris because the technology was discovered in Garreg Mach Monastery among Rhea's stash of secrets and what she used to create Sitri. Lindhardt improved it before it was stolen and used by Agatheans to create Iris. Olinda can manifest any crest for a short time but at a cost of her vital energy and is naturally crestless. Biologically she has two father’s Linhardt and Casper with the closest thing to a mother being Lysithea whose blood was used to give her, her unstable crest and in turn, Olinda's blood was used to cure Lysithea because of its crest-erasure properties.

Bonus Dimileth Kid