I think this ritual is performed both in the north and south. The Brahminical Pumsavana has been blended with basically a baby shower. I have heard that the ritual prays for a male child and also says "let female children be born elsewhere". Is this true?
It appears so. From the book Hindu Dharma: The Universal Way of Life by Swami Chandrasekarendra Saraswati:
The rites performed before a child is born are intended for the birth of a male child (niṣeka, puṃsavana, sīmanta).
Astronauts are kept in isolation before being sent up in space and after their return. Mantras have their own radiation that is even more powerful than what is found in space. If you appreciate this fact, you will understand why Brahmins are separated from the rest and special saṃskāras prescribed for them.
The body of a Brahmin (male) is involved in the nurturing of mantras. So from the time of conception itself it is to be made pure through saṃskāras like puṃsavana, sīmanta and so on. There are saṃskāras with the same objective also after the boy child is born.
As for 'female children be born elsewhere but not here', The Vivāha, the Hindu Marriage Saṁskāras quoting Sāṅkhāyana Gṛhyasūtra mentions such a verse but looks like it's uttered before the wife becomes pregnant.
Let him pound the root of the Adhyanda plant and sprinkle it at the time of her monthly period with the two verses, 'Speed away from here; a husband has she', with svaha at the end of each, into her right nostril. "The mouth of the Gandharva Visvavasu art thou' — with these words let him touch her, when he is about to cohabit with her. When he has finished, let him murmur, 'Into thy breath I put the sperm, N.N.!' or, 'As the earth is pregnant with Agni, as the heaven is with Indra pregnant, as Vayu dwells in the womb of the regions of the earth, thus I place an embryo into thy womb, N.N.!' or, 'May a male embryo enter the womb, as an arrow the quiver; may a man be born here, a son after ten months.' Give birth to a male child; may after him another male be born; their mother shalt thou be, of the born, and to others mayst thou give birth. 'In the male verily, in the man dwells the sperm; he shall pour it forth into the woman: thus has said Dhatar, thus Prajapati has said.' Prajapati has created him, Savitar has shaped him. Imparting birth of females to other women may he put here a man. 'From the auspicious sperms which the men produce for us, produce thou a son; be a well-breeding cow.' Roar, be strong, put into her an embryo, achieve it; a male, thou male, put into her; to generation we call thee. 'Open thy womb; take in the man's sperm; may a male child be begotten in the womb. Him thou shalt bear; having dwelt ten months in the womb may he be born, the most excellent of his kin.'
The full English translation of Sāṅkhāyana Gṛhyasūtra is available here.