Baby's Weight Gain in Pregnancy

Most babies fall into the 6- to 9-lb. range at birth, with the average being about 7 1/4 lbs., according to KidsGrowth.com. The baby’s weight gain is relatively slow during the first two trimesters. During this time, the baby is developing his organs, muscles and skeletal system. With regular ultrasounds, you will be able to see your baby’s growth and get an estimate of his weight gain throughout the pregnancy.

First Trimester

At the end of 13 weeks, you have officially finished your first trimester. Your baby now weighs about just 1 oz. Although you have probably gained a few pounds, this weight is not going to your baby just yet but to support the growth of the baby. Much of the few pounds you have gained is from the amniotic fluid, your growing uterus and increased breast tissue.

Second Trimester

The average baby weighs a little over 2 lbs. by the end of the second trimester (26 weeks). She gained about 16 to 20 oz. just in the previous three weeks. From here, her weight gain will continue to increase exponentially as she nears birth.

Third Trimester

As you near your due date, you will find yourself growing ever more uncomfortable. And this is for good reason. Your baby is now growing rapidly and becoming more tightly packed into your womb with each passing week. At the end of the 32nd week, average babies will weigh just 4 to 5 lbs. In the last eight weeks, he will gain an average of 8 oz. a week until he is born.

Slow Growth

The medical diagnosis for a fetus that is growing too slowly is fetal growth restriction. According to the American Pregnancy Association, these are babies who measure at less than 10 percent in the percentile chart for fetuses of the same gestation age. There are several causes including genetic defects, too little amniotic fluid, and using drugs, smoking or drinking alcohol during pregnancy.

Large Babies

Gestational diabetes and gaining too much weight during pregnancy can lead to having a larger than average baby. According to WebMD, research shows that women who gain more than 40 lbs. during pregnancy are at an increased risk for having a baby that weighs more than 9 lbs. Although some women have no trouble birthing a large baby, babies of this size are at an increased risk for getting stuck in the birth canal. This can lead to serious injury to the mom and baby.