There are many ways for an infant to crawl: classic (on all fours), commando (on the elbows), lopsided (on one side), or not at all. If Lucy happens not to crawl on schedule, some books will make you worry unnecessarily that she’s missing a milestone, is mentally delayed, or suffers from some other terrible condition.
The same books also suggest that you encourage her to crawl, although you’ll certainly feel silly down on all fours giving her crawling lessons. The babies who choose to crawl usually begin on average around the eight-month mark. But crawling is far from mandatory; many kids just skip it altogether and go straight to walking. If all other aspects of development are on par, there’s no need to worry if Lucy does not learn this optional mode of locomotion.