Lower back pain usually occurs in the first trimester of pregnancy and is pretty common.
A lot of times the pain can be one of the first signs of pregnancy.
Two most common causes of the pain is your expanding uterus and the hormonal changes.
The expanding uterus can cause a change in posture putting a strain on the lower back.
Hormones produced during pregnancy affect the back and can cause pain.
Another reason might be your spine might be out of alignment.
Ways To Alleviate Lower Back Pain

• A good way to alleviate the pain during your pregnancy is to pelvic tilts, they will strengthen the back and reduce the pain.
  Try to avoid lying flat on your back because the weight of your uterus may press on the vena cava.
  
• Start an exercise program to stretch and strengthen back muscles,legs, and abdomen.
  Swimming for example is a great way to strengthen the abdomen and lower back.
  The buoyancy takes the strain off your joints and back
• Avoid high heels and wear comfortable shoes such as flat heels.
  As your belly grows and your balance shifts, high heels will throw your posture even more out of whack and increasing your chances for pain.
  
• Posture is very important.
  Always stand upright.
  Pregnant women tend to slump their shoulders and arch their backs as their bellies get bigger, this can put an additional strain on the back.
  
• Avoid gaining too much weight.
  
• Another more uncommon way is to get chiropractic adjustments.
  This a great way to relieve the pain.
  Just make sure you find a chiropractor that specializes in pregnant woman