It is often assumed that informal unions are much less likely to produce children than traditional marriages, so that extension of the practice of unregistered cohabitation will have negative impact on the overall fertility. So advocates of traditional family life see its erosion as a cause of the recent fertility decline. But is fertility in fact so different in different types of union? To answer this question we analyze the average number of children born in the first union (first for the woman), which still make the chief contribution to the overall fertility. Thanks to the data came from the first wave of the Russian “Generations and Gender survey” (RusGGS-2004) we are able to compare fertility in three types of union: (1) unions, which began with official registration (about 50% of all first unions for women born in 1975-1979); (2) unions, which began with cohabitation, followed by marriage registration (about 40%); (3) informal unions, which remained unregistered (about 10%). As of today, unions, which started with marriage, and consensual unions, which were converted into marriage at a later date, are almost identical with respect to fertility for women aged 25 and 35. Nor was there ever any clear trend in differences between fertility for the two types of union in the past. From the point of view of completed fertility, official status of the union does not seem to have any significance in modern Russia, though a psychological sense of uncertainty about the relationship in case of unregistered unions may have negative impact on decisions about child-bearing. On the other hand, it could be that such unions are not registered and the relationships are more liable to threat of breakdown precisely because the partners cannot agree about having a child together.