Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 6

Chapter 6.—Concerning Twins of Different Sexes.

But even in the very conception of twins, which certainly occurs at the same moment in the case of both, it often happens that the one is conceived a male, and the other a female.  I know two of different sexes who are twins.  Both of them are alive, and in the flower of their age; and though they resemble each other in body, as far as difference of sex will permit, still they are very different in the whole scope and purpose of their lives (consideration being had of those differences which necessarily exist between the lives of males and females),—the one holding the office of a count, and being almost constantly away from home with the army in foreign service, the other never leaving her country’s soil, or her native district.  Still more,—and this is more incredible, if the destinies of the stars are to be believed in, though it is not wonderful if we consider the wills of men, and the free gifts of God,—he is married; she is a sacred virgin:  he has begotten a numerous offspring; she has never even married.  But is not the virtue of the horoscope very great?  I think I have said enough to show the absurdity of that.  But, say those astrologers, whatever be the virtue of the horoscope in other respects, it is certainly of significance with respect to birth.  But why not also with respect to conception, which takes place undoubtedly with one act of copulation?  And, indeed, so great is the force of nature, that after a woman has once conceived, she ceases to be liable to conception.  Or were they, perhaps, changed at birth, either he into a male, or she into a female, because of the difference in their horoscopes?  But, whilst it is not altogether absurd to say that certain sidereal influences have some power to cause differences in bodies alone,—as, for instance, we see that the seasons of the year come round by the approaching and receding of the sun, and that certain kinds of things are increased in size or diminished by the waxings and wanings of the moon, such as sea-urchins, oysters, and the wonderful tides of the ocean,—it does not follow that the wills of men are to be made subject to the position of the stars.  The astrologers, however, when they wish to bind our actions also to the constellations, only set us on investigating whether, even in these bodies, the changes may not be attributable to some other than a sidereal cause.  For what is there which more intimately concerns a body than its sex?  And yet, under the same position of the stars, twins of different sexes may be conceived.  Wherefore, what greater absurdity can be affirmed or believed than that the position of the stars, which was the same for both of them at the time of conception, could not cause that the one child should not have been of a different sex from her brother, with whom she had a common constellation, whilst the position of the stars which existed at the hour of their birth could cause that she should be separated from him by the great distance between marriage and holy virginity?