Twins: A Closer Look at Your Developing Babies

Parents.com partnered with the American Institute of Ultrasound Medicine (AIUM) to create this unique peak into a baby's development inside the womb. These images reveal all the intricate details of a baby's growth -- from a collection of cells to a full-term newborn. While most women may only receive one or two ultrasounds during pregnancy, which is normal, this slideshow of the 3rd trimester of pregnancy gives you a look at each week of development.

By Kristen J. Gough

American Institute of Ultrasound in Medicine - AIUM.org

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Week 6 (4 weeks from conception)

Fetal size: Length, 1/6 to 1/4 of an inch, crown to rump. Weight, still too small to measure.

Fetal development milestones: This week the babies are registering fetal heartbeats. Blood is pumping through their hearts. There are now two distinct amniotic sacs and the twins' heads, legs, and umbilical cords are forming.

What you're seeing: Although this may not look like much, these circles show two distinct gestational sacs -- twins! With a single pregnancy, only one sac would appear on the screen. Here, the sonographer (ultrasound technician) has identified each baby-to-be, one marked AA, the other BB. Most expectant mothers receive several ultrasounds during a twin pregnancy to ensure that both babies are growing properly and the pregnancy is progressing normally. Your unborn babies will grow at the same rate as with a single pregnancy -- but times two.

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