Srimad Bhagavatham : 5.3.18 & 19 :
Since I cannot find anyone equal to Me, I shall personally expand Myself into a plenary portion and thus advent Myself in the womb of Merudevi, the wife of Maharaja Nabhi, the son of Agnidhra.
This is an example of the omnipotence of the Supreme Personality of Godhead. Although He is one without a second, He expands Himself by svamsa, His personal expansion, and sometimes by vibhinnamsa, or His separated expansion. Lord Vishnu herein agrees to send His personal expansion as the son of Merudevi, the wife of Maharaja Nabhi, who is the son of Agnidhra. The rtvijah, the priests, knew that God is one, yet they prayed for the Supreme Lord to become the son of Maharaja Nabhi to let the world know that the Absolute Truth, the Supreme Personality of Godhead, is one without a second. When He incarnates, He expands Himself in different potencies.
Sukadeva Maharishi continued: After saying this, the Lord disappeared. The wife of King Nabhi, Queen Merudevi, was sitting by the side of her husband, and consequently she could hear everything the Supreme Lord had spoken.
According to the Vedic injunctions, one should perform sacrifices in the company of one’s own wife. Sapatniko dharmam acaret: religious rituals should be performed with one’s wife; therefore Maharaja Nabhi conducted his great sacrifice with his wife by his side.