Month by month fetal development of twins in the womb.

How do twins develop and change throughout the prenatal period? According to the American College of Obstetricians and Gynecologists, renowned medical illustrator Peg Gerrity provides a glimpse into the womb to demonstrate the development of fraternal twins from implantation until they are ready for birth.

At three weeks, nonidentical twins, also known as fraternal or dizygotic twins, are the most common type. Each baby develops from a separate fertilized egg. By this time, the zygote has developed into a cluster of cells called a blastocyst, which begins to implant itself in the uterine wall.

By four weeks, the twins have transformed into embryos, consisting of two layers of cells that will give rise to all the body parts and organs. Within the next week, each baby forms its own amniotic sac and placenta.

At eight weeks, the babies already have flexible hands and feet. Their hearts have been beating for two weeks, and nerve cells in their brains are growing to establish connections.

When reaching 12 weeks, the twins become more active, kicking and stretching. Their hands meet on their chests, and tiny nails start to grow. Eyelids develop to cover their eyes, and tooth buds emerge in their gums.

By 16 weeks, the babies already have distinctive fingerprints. They can squirm and even urinate in the amniotic fluid. They begin to drink and swallow the fluid, and their gender can be determined through ultrasound.

At 20 weeks, it is possible to communicate with the twins as they can hear sounds. Hair starts growing on their buttocks, and a layer of white fat appears on their skin, providing protection in the liquid environment.

At 24 weeks, their eyebrows start to grow, and tooth buds and taste buds are fully formed. The skin becomes red and wrinkled, irrespective of the babies’ skin color after birth.

By 28 weeks, the twins can open their eyes, and their eyelids have eyelashes. Fat layers accumulate under their skin, giving it a smoother appearance. They can suck their fingers, and you may feel them when they experience hiccups.

At 32 weeks, the nails cover their fingers and toes. The development of their hands, feet, and trunk continues to improve. Many twins are expected to be born within the following weeks.

Most twins are typically born around 36 weeks. The lungs mature by around 34 weeks, and during the final weeks, they gain weight. The average weight for most twins at this stage is approximately 2500 grams.

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