To better understand the changes your body goes through during the last trimester and labor, it is helpful to be familiar with basic anatomy. The illustrations on the right show a woman's body before pregnancy and at 37 weeks.

Changes to note:

  • Breasts grow in preparation for breastfeeding
  • Uterus grows to a much larger size to hold the baby
  • Lungs and diaphragm are compressed, which can cause shortness of breath
  • Stomach compression can make you feel full faster and contributes to heartburn
  • Pressure on the bladder often causes frequent urination
open activity Activity: Comparative Anatomy


Comparative Anatomy

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play video Anatomy: Take a 3D look inside pregnancy.

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view text Take a 3D look inside pregnancy.

To understand the discomforts of late pregnancy and the changes that will take place during labor, it's helpful to be familiar with some basic anatomy. This animation shows a pregnant woman with a full-term baby positioned head-down in preparation for birth. The baby lives inside of her mother's uterus, a strong balloon-shaped muscle; when it tightens, it causes a contraction. The bottom part of the uterus is the cervix. A mucous plug fills the cervix to protect the baby from the outside environment. Below this is the vagina, which will be your baby's passageway into the world. The placenta is an organ created by a woman's body just for pregnancy that grows into the wall of the uterus. The placenta produces pregnancy hormones, transfers oxygen and nutrients from mother to baby, and removes waste products from the baby's blood. The baby's blood travels to and from the placenta through the umbilical cord. Growing out from the placenta is the bag of waters, or amniotic sac. The water, or amniotic fluid, inside the sac cushions the baby in a comfortable, warm environment.

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Fetal Development: These amazing 4D ultrasound images show you how babies move in the womb.

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Fetal Development
view text These amazing 4D ultrasound images show you how babies move in the womb.

The third trimester starts at week 29, lasts 12 weeks, and ends with the birth of your baby at around 40 weeks. As it progresses, the growth of your abdomen is accompanied by your baby's development. 4D ultrasound images like this one give us a special glimpse at what babies can do in utero during the third trimester. As this baby rubs her eyes, you can see that her facial features are already unique. In this next ultrasound, you can see this baby lick his arm. His mother probably just ate a sugary snack that he's tasting. In this one, the baby's practicing how to move her hands and fingers. Perhaps she'll give mom a little jab by accident. While babies move around and explore the womb, important fetal development continues right up until birth: their lungs mature, their brains are wired for thinking, and they gain the body fat needed to stay warm outside of the womb.

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