When we cling to anything, we identify with it. We make ourselves afresh in its image; in a sense we become that object, at least momentarily. This is the process of becoming, fueled by craving and clinging by which we come into existence moment after moment.
The image here is of a pregnant woman. There could be no apter symbol of the process of becoming than this; from an act of sensual desire the process of growth inevitably unfolds.
Existence is not a state, it is a process. This coming-into-being we experience all the time, as we constantly reform ourselves in line with our desires and our actions. This is the fruition of the process we started when we made sense contact and it arose feelings which we craved and then clung to.
This stage represents the passive aspect of karma, the resultant. Here is the culmination of the active aspect of the second stage (karmic formations.)