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Everywhere I look describes how the 5 sacral vertebrae start fusing together during adolescence and are totally fused by the time you're 30 or so. But nobody seems to explain why this happens.

It seems like it would be disadvantageous for women, since presumably having a fused sacrum makes giving birth more difficult- apparently post-partum fractures are not uncommon (http://rheumatology.oxfordjournals.org/content/42/2/384.long). So why does it happen?

EDIT: Matthew McTernan gave a great answer, but I am greedy and would like to know more: Why does the sacrum only start fusing together in adolescence, and then finish in your late 20s - early 30s? Most major development ends in your early 20s. Does this have something to do with reproductive concerns? If so, why does it happen on the same timescale in men and women?

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It isn't just humans that have fused sacral vertebrae. Animals on the phylogenetic tree ranging from Squamata (lizards), to Aves (birds), to even Monotremata (egg-laying mammals) all have fused sacral vertebrae. The fused vertebrae can serve as a stronger, weight-baring structures for animals that do land locomotion, including humans. Any bones that have evolved to withstand extensive force will also be thick or fused together: and example is the fusion of the radius and ulna in frogs. This fusion of bones allows for the forearms to withstand the force of landing after a large leap.

Photo references:

Bird Sacrum: http://www.discussfossils.com/forum/uploads/842/sacrum_2.jpg

Frog Radioulna: http://classconnection.s3.amazonaws.com/573/flashcards/4420573/png/appendicular_skeleton-frog-142D89D722717E64724.png

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