Arika is confirmed to be pregnant after her fall
Rietta's last event confirmed she's been pregnant since her first time with Gurú
Terena is the only one with no confirmed pregnancy
I believe in Terenas case, it's because of the worms/bugs in her, as removing them early could ruin her body/womb.
Surprisingly since she is the first one to leak out breast milk, which normaly first happens when pregnant, already in EV29.
Arika first started at the end of EV42 when she finally fell and got pregnant at the same time.
Rietta first leaked out breast milk when banging that brat in EV37, and that is about two days after (at least, depending the order the player chooses) she lost her virginity on EV31.
Of course, Terena could still get pregnant before the worms/bugs are removed, but that is up to TH to deside.