When do babies crawl?

To answer this question, it's important to address several factors.
Consider the Source:
Depending upon where you obtain your information, you'll receive a variety of responses ranging from 6 to 12 months. If you read the "what to expect" book, which most parents have, you'll find the range between 6 and 9 months. However, this answer was based on a time prior to the "back to sleep" campaign. Since 1993 babies have spent less time on their tummies and thus the crawling range has been considerably altered. If you look to the experts; AAP guidelines (American Academy of Pediatrics), the answer is between 8 and 12 months.
Your Expectations:
It's also important to break this answer down so parents have a better understanding of crawling and the expectations for crawling:

• Approximately 50% of babies do some form of crawling* by 7 months
• About 75% by 10 months
• 90% by 12 months

"Some form of crawling" means; bear crawl - hands & feet, military drag - using forearms and dragging body, One-legged - scooting around in a half-sitting position with one leg tucked under and one leg positioned on knee, downward-facing dog-yoga style position where bum is high in the air & baby is on hands & feet, or traditional crawl - on hands & knees.
All Babies are Unique!
Every Baby will master each developmental skill uniquely and in their own time. Some babies are content to sit and play with toys placed in front of them. Others seem to be driven to explore. Some babies are reserved and prefer to be observers. It is so important to spend time just playing with your baby, getting to know him/her and appreciating who they are and what motivates them.
Understand the Times. 
It's also important to understand babies spend far less time on their tummies than they used to. Prior to the "Back to Sleep" campaign, parents placed their babies on their stomachs to play, for exercise, and even to sleep. Now we know that it's best not to have baby sleep on their stomachs and some parents are even reluctant to give baby that very important "tummy time." As a baby grows and is able to roll over it becomes almost impossible to keep them on their stomachs. However, in order to crawl, babies must have "tummy time" so they are capable of building their upper body strength and can develop the ability to reach and grasp for objects in front of them.
Know your Options!
In the WeePlay Crawlers Developmental classes of Tampa, parents learn so many different "tummy-time" alternatives that most babies need very little traditional "tummy-time." Once parents and caregivers learn the WeePlay PT's (positions & toys), crawling is only a matter of weeks away!

Miss Tammy