Deuteronomy 21:17

21:17 Rather, he must acknowledge the son of the less loved wife as firstborn and give him the double portion of all he has, for that son is the beginning of his father’s procreative power – to him should go the right of the firstborn.

Psalms 78:51

78:51 He struck down all the firstborn in Egypt,

the firstfruits of their reproductive power in the tents of Ham.

Psalms 105:36

105:36 He struck down all the firstborn in their land,

the firstfruits of their reproductive power.