prajapatir nama tayor akarsid
yah prak sva-dehad yamayor ajayata
tam vai hiranyakasipum viduh praja
yam tam hiranyaksam asuta sagratah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Prajapati Kasyapa, the creator of the living entities, gave his twin sons their names; the one who was born first he named Hiranyaksa, and the one who was first conceived by Diti he named Hiranyakasipu.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There is an authoritative Vedic literature called Pinda-siddhi in which the scientific understanding of pregnancy is very nicely described. It is stated that when the male secretion enters the menstrual flux in the uterus in two successive drops, the mother develops two embryos in her womb, and she brings forth twins in a reverse order to that in which they were first conceived; the child conceived first is born later, and the one conceived later is brought forth first. The first child conceived in the womb lives behind the second child, so when birth takes place the second child appears first, and the first child appears second. In this case it is understood that Hiranyaksa, the second child conceived, was delivered first, whereas Hiranyakasipu, the child who was behind him, having been conceived first, was born second.