The First Trimester of Pregnancy: The Developing Baby Health Article

Weeks Twenty-Six through Twenty-Nine

During weeks 26 to 29, the baby's lungs mature so that they will be able to take in oxygen and release carbon dioxide once the baby is born. With each breath, amniotic fluid is taken into the lungs, which stimulates the lungs to develop. The bone marrow inside the bones of the fetus takes over the job of making blood cells from the liver and the spleen.

By this time, an ultrasound may show the baby breathing, its eyes moving, and the baby sucking its thumb and sticking its tongue out.

Weeks Thirty through Thirty-Four

During weeks 30 to 34, the fetus continues to gain weight and build up fat. Skin becomes pink and smooth. Most fetuses will turn to a head down position during this time, if they have not done so already.

Weeks Thirty-Five through Thirty-Eight

During the last month (weeks 35 to 38), growth slows and the baby's abdomen is about as large as the head. At this time, the baby is considered "full term."

The estimated date of delivery (EDD) is 266 days from conception or 280 days from the beginning of the last menstrual period (LMP), that is 38 weeks from conception or 40 weeks from LMP. Most mothers start labor during this time, but a small percentage of women deliver later. Babies born at full term generally have mature lungs and can breathe without difficulty.

Go to The First Trimester of Pregnancy: The Pregnant Father and The First Trimester of Pregnancy: Mother.