http://revach.net/article.php?id=3417

Section: Taryag Mitzvos   Category: Minchas Chinuch
Pru U'Rivu - Children From A Different World

The Shulchan Aruch states that a Ger who had a son and a daughter who became Geirim along with him, has fulfilled the Mitzvah of Pru Urivu, since while he was a non-Jew they are considered his children.  However the rule is  that if someone does a Mitzvah during a time that he was Patur from the Mitzvah he has not fulfilled the Mitzvah. If so,asks the Shaagas Aryeh in Sefer Turei Even, how can the children that he had while he was a non-Jew be a fulfillment of the Mitzvah of Pru Urivu, if a non-Jew is not obligated in the Mitzvah of Pru Urivu? 

The Minchas Chinuch answers that Pru Urivu is an ongoing Mitzvah that is fulfilled as long as one has children. The Mitzvah of Pru Urivu isn’t the act of conceiving a child.  Conceiving a child is only a Hechsher Mitzvah (preparation for the Mitzvah). The Mitzvah is being fulfilled every second that the children are alive. Therefore if the children die one no longer is fulfilling the Mitzvah and is obligated to have more children. Therefore even though a Ger had the children while he was still a non-Jew and was Patur from the Mitzvah, since the children are still alive after he became a Ger he is fulfilling the Mitzvah while he is a Jew.