Twins form in one of two ways: 

  
Identical twins occur when a single fertilized egg splits into two. Identical twins look almost exactly alike and share the exact same genes. Most identical twins happen by chance. 

One

Two

  
Fraternal twins occur when two, separate eggs are fertilized by two, separate sperm. Fraternal twins do not share the exact same genes — they are no more alike than they are to their siblings from different pregnancies. Fraternal twins tend to run in some families. 
Multiple births can be fraternal, identical, or a combination. Multiples associated with fertility treatments are mainly fraternal. 

Pregnancy with multiples 
 
Years ago, most twins came as a surprise. Now, thanks to advances in prenatal care, most women learn about multiple pregnancies early. You might suspect you are pregnant with multiples if you have more severe body changes, including: 
•    Rapid weight gain in the first trimester 
•    Intense nausea and vomiting 
•    Extreme breast tenderness 
Your doctor can confirm whether you are carrying more than one baby through ultrasound. If you are pregnant with twins or other multiples, you will need to see your doctor more often than women who are carrying only one baby because your risk of complications is greater. Women carrying more than one baby are at higher risk of: 
•    Preterm birth 
•    Low birth weight 
•    Preeclampsia 
•    Gestational diabetes 
•    Cesarean birth 
More frequent prenatal visits help your doctor to monitor your and your babies' health. Your doctor will also tell you how much weight to gain if you need to take extra vitamins, and how much activity is safe. With close monitoring, your babies will have the best chance of being born near term and at a healthy weight. 
After delivery and once your babies come home, you may feel overwhelmed and exhausted. Ask for help from your partner, family, and friends. Volunteer help and support groups for parents of multiples also can ease the transition.