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Development of a Baby in the Womb

By Sharon Szentpetery, Ph.D. ; Updated June 13, 2017

An unborn baby goes through a series of coordinated, rapid developments while in the womb. From a single fertilized egg, the growing embryo forms all of the body organs in just a few weeks. Within the first few months, the baby begins to move, taste, see and hear. By birth, the infant knows her mother’s voice and is prepared for her first breath of air.

A Growing, Nurturing Environment

Development begins as soon as the egg is fertilized by the sperm. The fertilized egg immediately starts to grow and divide, forming a group of cells that will later become the embryo. About 6 days after fertilization, the cells attach to the uterus lining. Over the next couple days, they become firmly embedded in the lining -- this process is called implantation.

The growing embryo initially receives nourishment from the cells lining the uterus, until the placenta takes over. The placenta is made up of tissues from both the mother and the developing baby. The mother’s blood flows into the placenta and blood from the placenta travels via the umbilical cord to the baby, providing oxygen and nourishment while carrying away waste products.

Body Organs

As the embryo continues to grow, working organ systems develop. By the 3rd week after the egg is fertilized, a fold of the embryo becomes the neural tube, which will continue to develop into the brain and spinal cord. The circulatory system develops quickly, producing blood cells that are pumped through the 4-week-old embryo at a rate of approximately 105 to 120 beats per minute. By 5 weeks, the baby has a layer of skin that is only 1 cell thick. This extremely thin skin is transparent, revealing rapidly developing organs, including the lungs, liver, stomach, intestines, kidneys, eyes and sex organs, as well as budding arms and legs.

Movements and Sensations

By 7 weeks, the ends of the hands are separating into individual fingers, and toes are forming on the feet. Though the mother may not yet feel them, the baby makes her first small movements at this time. The baby can sense touch and will become more active if someone presses firmly on the uterus. By 10 weeks, an ultrasound may reveal the baby's sex and catch her waving, stretching, sighing, swallowing or hiccuping. At approximately 20 weeks, the baby opens her eyes in the womb. After 20 weeks, babies can respond to sound, and by 32 weeks, they appear to be able to recognize their mother’s voice.

Facial Expressions

Babies between 24 and 36 weeks may sometimes be seen smiling and scowling in the womb on ultrasound. While babies can feel pain at this age, investigators are not yet clear on the significance of facial expressions in the womb.

Ready to Breathe

In the last few weeks, the baby is getting ready for her first breath and life outside the womb. Though the baby can practice the movements of breathing while inside the womb, air sacs in the lungs are not fully developed until 32 weeks. The developing lungs are filled with fluid that is reabsorbed before and during birth so that the newly born infant can take her first breaths of air -- and give a good cry to let her mom and dad know that she has arrived.

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