Dear Rabbi Kornfeld:
I was struck by the symmetry of the number of days that a birth mother is considered to be in sakana, 3, 7 and 30; and the number of days in the mourning period (aveilus), 3, 7 and 30. Is this comparison discussed in any sources you've seen, and what is said about it.
The commentary attributed to Rashi in Divrei ha'Yamim I 16:34 explains that one acts the same way with great Simchah as he acts out of great grief. This may explain the symmetry regarding the various levels of rejoicing of a Chasan and Kallah after their marriage (7 days, 30 days, and one year) with the levels of mourning (7, 30, and one year).
Contemporary Musar writers have also expounded upon the meaning of this symmetry (I will try to find exact sources for you). We do not know of any source that addresses the particular symmetry which you noticed, although it could be explained along the same lines as the symmetry between marriage and Aveilus.