Multiple births—including twin births--are at an all-time high in the U. S. According to the Center for Disease Control the number of twin births in 2005 at the time of the last consensus was 133,122. So what is driving the twin pregnancy trend? More women are having babies after age 30 and more are taking fertility drugs. Both boost the chance of carrying more than one baby.

Age-related hormonal changes in women may cause more than one egg to be released at a time, which boosts the odds of conceiving fraternal twins. Also, older women are more likely to take fertility drugs or use other fertility technologies like in vitro fertilization to conceive. A family history of twins also makes multiples more likely. For more information on age-related hormones and other risks of pregnancy read Women’s Health, Fitness and Shape magazines.

Fraternal or non-identical twins are the most common. Fraternal twins biologically occur when two separate eggs are fertilized by two different sperm. This results in each twin having his or her own placenta and amniotic sac. Genetically fraternal twins are no more alike each other than any other siblings and can be two girls, two boys or a boy and a girl.

Identical twins biologically occur when one fertilized egg splits and develops into two fetuses. These fetuses may share a placenta but usually has separate amniotic sacs. Identical twins are genetically identical.

Recently a handful of celebrities have given birth to twins: Angelina Jolie, Jennifer Lopez, Julia Roberts and Marcia Cross. Actress-model Rebecca Romijn and her husband actor Jerry O’Connell just announced in People magazine they are expecting twins in the winter; Lisa Marie Presley also just announced she is expecting twins. Other celebrities who are parents are twins are Denzel Washington, Sean Combs and Cybill Shepherd. President and Mrs. Bush are the parents of twin daughters Barbara and Jenna Bush.

There are and have always been many famous twins in Hollywood. Often television and film production companies hire twin babies and children to share one role of a child character. The production companies do this so that child-labor laws, which require children to work shorter hours than adults, will not slow down production. Famously, twins Erin and Diane Murphy shared the role of "Tabitha" on the 1960s fantasy sitcom "Bewitched," Lindsay and Sydney Greenbush shared the role of "Carrie" on the 1970s drama "Little House on the Prairie" and twins Mary-Kate and Ashley Olsen shared the role of "Michelle" on the 1980s sitcom "Full House." Many twins including the Olsens and Tia and Tamara Mowry have starred as twins in family movies and television shows.

Sometimes only one twin is famous. Some examples of this situation are race car driver Mario Andretti (brother Aldo), actress Scarlett Johanssson (brother Hunter), model Gisele Bundchen (fraternal sister Patricia), actor Vin Diesel (Paul), singer Alanis Morissette (Wade) and actress Jill Hennessy (Jacqueline). For more famous twins read People Magazine, People En Espanol, Star magazine and In Touch Weekly.

Every year in Twinsburg, Ohio the city celebrates "Twin Days" in celebration of twins. For the upcoming 2008 event, which will be the 33rd annual Twin Days, event planners are hoping for 3,000 sets of twins.

In a comedic nod to twins, Arnold Schwarzenegger and Danny DeVito paired up as twins in the 1988 movie called Twins. For more movies and television shows about twins or featuring twins read Entertainment Weekly, People magazine and O The Oprah Magazine.