Hormonal Changes

• Fluctuating hormone levels during pregnancy can at least partially account for mood swings. Your body is suddenly experiencing higher levels of hormones like estrogen and progesterone, and the sudden shifts can be a shock to your system. Until your body adjusts, you may find yourself reacting to everyday events differently than you used to.
  
  

Physical Stress

• Pregnancy is a long and physically stressful period for any woman, and the physical changes you undergo may make you irritable. Morning sickness, tender breasts and nipples, sore and swollen feet, tiredness and general discomfort take a serious toll on your body, and the changes may be enough to make you prone to bouts of anger.
  
  

Worry

• No one can predict the outcome of her pregnancy and her child's life with complete accuracy, and this can be an unnerving reality. You may find yourself worried about pregnancy complications, money issues and a lack of confidence as your due date approaches. Even the thought of physically giving birth may be enough to stress you out. All these stressors can contribute to irritability during pregnancy.
  
  

Isolation

• As you carry your child for nine months, you may feel helpless in your condition. You may feel that given your condition, people have unfair expectations of you. If you are able, ask your partner for extra help with tasks like housework---you may need to explain that you need some extra "me time" to cope with everything that is going on.
  
  

Insomnia

• One potential side effect of pregnancy is insomnia, or lack of sleep. Not getting enough sleep deprives you of a chance to physically, mentally and emotionally rest and recharge. Combined with the sudden and extreme physical effects of pregnancy, a lack of sleep can cause major irritability.