Periodically, Mercury appears to slow down, then stop, (stationary retrograde) and then move slowly backwards for several weeks (retrograde).  Eventually, it appears to stop again and reverse direction (stationary direct) moving slowly forward once again (direct). Eventually Mercury appears to return to its normal orbital speed.

This occurs because Mercury travels faster than the Earth, and it periodically catches up with Earth, passing us by. When Mercury "goes retrograde" it does not actually slow down, stop and move backwards. It only appears to do so. This phenomenon occurs near Mercury's inferior conjunction to the Sun—as Mercury passes closest to the Earth, between the Earth and the Sun. This phenomenon has to do with the relative speed of the Earth and Mercury as well as their relationship to each other at a particular point in their orbits. There are three Mercury Retrograde periods each year, each lasting approximately 3 weeks.

What does Mercury Retrograde mean?
In mythology, Mercury is the messenger of the gods.  Astrologically, the planet Mercury is said to be associated with or to rule all activities that relate to communications, travel, learning such as:

• signing agreements
• classes and schools
• sending correspondence
• communications equipment such as computers, radios, and phones
• traveling and travel equipment such as cars, bicycles, trains, and planes

Classically, Mercury retrograde periods are not considered ideal
for engaging in Mercury related activities such as:

• placing ads
• making important decisions
• travel
• purchasing communications equipment
• signing contracts
• initiating business deals
• sending important correspondence or any type of message
• starting school or any important project
• beginning any new enterprise (as plans may be reversed later)

A Mercury retrograde period is thought to be excellent for:

• reviewing and revising plans
• catching up on old business
• cleaning out the metaphorical closet
• deeply considering issues

However...
like everything else in life, these rules are not steadfast and your experiences may differ from these generalities. Never let the planets rule your life! If you simply must sign a contract or begin a new job when Mercury is retrograde, go for it!

While some of us are very sensitive to Mercury cycles and find that certain activities become difficult during retrograde periods, while others experience no such problems. How you experience Mercury has a lot to do with planetary placements at your birth, as well as with current planetary positions. The best way to understand how Mercury retrograde periods impact you is to track your own experience. Check the dates that Mercury is retrograde to track how sensitive you are to this cycle.

My baby might be born during Mercury retrograde and I'm nervous with how this will affect him/her in life?
Do not worry about your little baby if s/he's born with a retrograde Mercury. Many very intelligent people have that in their charts. It's the mark of a deep thinker and, depending on the sign and placement of Mercury in relation to the natal chart as a whole, it can indicate an individualistic thinker—someone who absorbs information from external sources, and processes it coming up with their own conclusion. The Mercury retrograde native does not approach things in a superficial way, and often has an uncanny ability to see beneath the surface of issues.

Again, depending on the sign and placement, it may indicate a person who is hesitant to share opinions unless they feel confident that others want to know their thoughts. So if your baby has Mercury retrograde, be sure to encourage him/her to let you know what s/he thinks—and to give him/her time to consider options. So you might say to a young child with this placement, "Do you want milk or orange juice for breakfast? Think about it while I butter your toast and let me know."