After a woman becomes pregnant, the fetus gradually grows in the womb. The uterus, which originally resembled a pear-shaped inverted uterus, became the size of a watermelon at full-term pregnancy. The weight of the uterus also increases from 50 grams in non-pregnancy to 1000 grams in full-term pregnancy. As the uterus and fetus grow, the sleeping position of pregnant women becomes more and more important, especially in the third trimester of pregnancy.