Nine months of pregnancy may seem eternally long to some expectant mothers, but in reality, your baby is growing at a rapid rate. By the end of just the 12th week of pregnancy, your baby is fully formed and has all of her organs in place. In fact, throughout the course of the pregnancy, your baby will grow from being 0.1 mm long to an average of more than 19 inches long.

Time Frame

Pregnancies are divided up into trimesters. During the first trimester, or the first 12 weeks, the fetus is growing extremely rapidly. In this short amount of time, the baby goes from a fertilized egg to a tiny body with formed organs. During the second trimester, the organs continue to develop and mature, tissues and the body’s systems are refined, and the baby develops fingernails, earlobes and other details. The third trimester features hair growth, the development of body fat and, most importantly, maturity of the lungs.

Size

A fertilized egg is too tiny to be seen with the naked eye. On average, it is between 0.1 and 0.2 mm. By the end of the first trimester, the baby is around 3 inches long. By the end of the 6th month of pregnancy, the baby has reached an average length of 14 inches and weighs around 2 lbs. Healthy, full-term babies can range in weight from a little under 6 lbs. to as much as 11 or more pounds, with an average length of just over 19 inches.

Milestones

Listening to your baby’s heartbeat is perhaps one of the biggest milestones in the development of your baby’s body. A baby’s heart beats as early as the 10th week of pregnancy, according to Sutter Health, but it usually isn’t heard until around the 12th week of pregnancy. Doctors use an instrument called a Doppler to bounce sound waves off your baby’s beating heart, allowing you to listen to it. At around 18 weeks of pregnancy, your baby’s body is developed enough to see if it is a boy or girl. During the sixth month of pregnancy, your baby’s eyes open and she may even develop hiccups. By the end of the seventh month, most babies can survive outside the womb, although they will need special care.

Warning

Babies that measure smaller than expected are not usually in danger, but there are some serious complications that can slow the growth of your baby. The most dangerous is low amniotic fluid, or oligohydramnios. This is diagnosed with an ultrasound. Low amniotic fluid can be caused by a leak in the membranes, a problem with the placenta, a condition such as gestational diabetes, or a problem with the baby itself. If the conditions is noticed late in the pregnancy, you might be encouraged to deliver your baby early. If the condition is noticed before the baby can survive outside of the womb, you will most likely be put on bed rest and monitored closely.

Considerations

Many women worry about the size of their baby. Will he be “too big” to be born? In most cases, the answer is “no.” Babies who weigh more than 9 lbs., 15 oz. are considered larger than average, or “macrosomic,” according to BabyCenter. If the doctor is worried, she might suggest a cesarean section, but this is rarely done just because the baby is larger than normal. Cesarean sections for this reason are usually only performed if the mother’s pelvis is abnormally shaped or very small.