Vyasasmriti 1.16

1.16

vivāho mantratastasyāḥ śūdrasyāmantrato daśa
garbhādhānaṃ prathamatastṛtīye māsi puṃsavaḥ

Her marriage is performed with mantras; for a Śūdra, ten rites are performed without mantras.
The rite of conception comes first; in the third month, the rite for quickening a male child;