pregnant with eren's child . i mean you need a sacrifice that have royal blood . which mean the father should be either eren or zeke , but paradis side are supposed to distrust zeke since his past action taken against paradis . eren on the other hand is not only fulfilling the condition but also trustworthy as he is their hero , not to mention he is currently holding three titan power (attack titan & founding titan (inherited from his father) also recently war hammer titan (inherited from marleyan) .) his child will probably have the necessary genes to adapt to founding titan's power like the previous holder before grisha jaeger snatched them from reiss royal family . zeke on the other hand won't have that gene and have time less time to make her pregnant since he is imprisoned and waiting to be eaten in order to pass the torch to beast titan's inheritor . tl;dr eren have 3 factor that zeke don't have : founding titan , time , and trust .