Pregnancy is definitely an exciting time in a woman's life – but discovering that twins have been implanted in your uterus is unknown territory for many women. Instead of being responsible for carrying one baby, the well-being and development of two embryos is now at stake.

Dr. Lillian Schapiro, an OB/GYN and women's health expert with Peachtree Women's Specialists, P.C. in Atlanta, Ga., knows all about carrying twins. After five cycles of fertility treatments, she became pregnant with twin daughters and delivered them in 1996. Her experiences inspired her self-published novel, Tick Tock (iUniverse, 2005), where the main character, a female gynecologist in the midst of a fertility rotation, discovers she may be infertile herself. She gives us a look at what really goes on in the womb during the development of twins.

Fraternal twins, which are the result of two separate fertilization events, are genetic and also the result of in vitro fertilization and assisted reproduction, Dr. Schapiro says. Identical twins occur naturally during conception and are formed from a single embryo whose cells dissociated from each other.

Most women discover they are expecting twins during the first ultrasound when two yolk sacs become visible. From there, several different things can happen as the embryos develop, Dr. Schapiro says. While identical twins usually grow within separate amniotic sacs, they sometimes share the same placenta. Fraternal twins have separate placentas and amniotic sacs.

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