There is 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.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Third Canto, Chapter 17 – Text 18