Back pain is the most common orthopedic complaint in most of the pregnant women. According to the scientific literature 50% to 80% of the pregnant women complain of back pain at sometime during their pregnancy. Out of the different back pains it is the lower back pain which most of the pregnant mothers suffer.
Number of factors can be attributed for the cause of low back pain in the pregnancy. Hormonal and postural changes play a major role in the cause.
During pregnancy many hormones are produced out of which Relaxin is the one which is of our concern. The function of hormone Relaxin is to loosen the joints and ligaments in your body. This helps to relax the ligaments in the pelvis which in turn makes room for your baby inside and also helps during delivery of your baby.
Since the hormone have no selective role in relaxing the ligaments and joints of the pelvis only, all the ligaments and joints in the body are stretched. Laxity of the ligaments which support the spine results in the instability which can cause back pain. 
As the baby grows and size of the uterus expands, the posture of your body is altered resulting in forward bending of lower spine and compensatory backward bending of upper spine. Abdomen shifts forwards and downwards altering the normal biomechanics of the spine. This in turn adds on strain to back muscles which may become sore and tired.
Two types of lower back pain are observed during pregnancy,
1. Lumbar pain
2. Posterior pelvic pain
Lumbar pain
It is similar to the kind of back pain that you may have experienced before the pregnancy. It is caused by sitting or standing for prolonged periods. It presents as pain in the lower back, just slightly above the waist line. 
Posterior pelvic pain
It is the pain experienced at the back of the pelvis, below the waist line, across the buttocks and near the tail bone. It is the most common type experienced by pregnant women. It can be aggravated by bending twisting, climbing stairs, or leaning forwards.



Management
Back pain during pregnancy could be controlled and prevented by taking necessary precautions and following regular exercise protocol. Exercises help to strengthen, stretch the back muscle. These include pelvic tilt exercises, back stretches, hamstring stretches and kegel exercises.
Walking and swimming are the two preferred exercises during pregnancy to maintain the tone of back muscles. Walking helps to maintain flexibility of muscles of lower back, hips and knee. Swimming and other aquatic exercises allows full body work out with minimal stress and strain to the body. Prenatal yoga or pregnancy yoga helps to stretch and strengthen the back muscles and aid in relieving the pain. 
It is not only exercises which help in controlling the back pain, but maintaining proper posture while standing and sitting will help. When you stand try to tuck your hips and pelvis, try resting one foot on a stool. Prolonged sitting can hurt you back, so take frequent breaks if you plan to sit for long times. Safe lifting and bending practices help you to prevent undue strain on your back. When bending over to lift any object, bend at the knees instead at waist and use your thigh muscles to push yourself up instead of back muscles. Try not to lift heavy objects. Wearing low heeled shoes with good arch support may help to alleviate back pain. 
Sleeping posture most favorable for pregnant women is to sleep sideways, preferably on left side. Try sleeping with your knees and hip bent, with pillows between knees and one under the belly.
Note:
If you are pregnant with sever back pain, which is rhythmic and feels like menstrual cramps consult your doctor. If you have back pain with numbness or weakness in both lower limbs it needs urgent evaluation by your doctor. Dull back pain during late stages of pregnancy could be signs of preterm labour.