There are many different kinds of twins, including fraternal twins (two eggs and two sperm), identical twins (one egg and one sperm), mirror image identical twins (separation on day 5 of development), identical twins with two placentas, one placenta and two sacs, one placenta and one sac, and finally conjoined identical twins (separation after day 10 with the fetuses attached), always with one placenta and one sac.

Overview of anatomy

Uterus, fallopian tube and ovary. Shows steps of ovulation, fertilization and early development. Most twinning occurs between first 30 hours and 5-6 days. From Langman’s Medical Embryology.

How identical twins can be formed

A: divides at two cell stage (most identical). B: divides at about 5 days, creates mirror image identical twins (see placenta below). C: divides around day 9, mirror image twins, but now only one sac (dangerous because cords can become tangled). If division happens after day 10, create conjoined twins (see xray below). From Langman’s Medical Embryology.

Monochorionic diamniotic twin placenta

Monochorionic diamnionic twin placenta

Monochorionic diamnionic twin placenta, like column B above. ©Harvey Kliman, MD, PhD

Conjoined twins

Conjoined twins, 18 weeks, xray. ©Harvey Kliman, MD, PhD

Fertilization and two-cell embryo

Detail of fertilization and then two cell stage where the cells are already starting to be different (shown as red versus green cells). The red cell has different genes turned on compared to the green cell. ©Harvey Kliman, MD, PhD

Mirror-image identical twins

Diagram showing how the two cell embryo with one red and one green cell makes a ball of cells, half red, half green. If this grape-like cluster splits down the middle into one red half and one green half, each half will make a baby with different gene expression, even though they are "identical twins" (red diaper versus green diaper). ©Harvey Kliman, MD, PhD

Sarah & Amanda: Identical Twins at Last

7 months

Sarah and Amanda's parents were told that their twin daughters were not identical at the time of their birth, however as they grew up they certainly looked like identical twins. After hearing a lecture at the Yale University School of Nursing given by Dr. Kliman, Sarah asked if there was a way to find out one way or another. Here is their story of discovery.

3 year olds

Sarah and Amanda eye-to-eye

4 years

Amanda and Sarah at 4 years

Sarah and Amanda waiting to get their blood drawn

Sarah convinced Amanda to come to New Haven to have her blood tested.

Blood draws

Sarah, left, the nursing student, doesn't want to look. Amanda is relaxed.

Blood typing results

The big day arrived and the results are in: Sarah and Amanda are identical twins!

7/1/2005: Twins: Identical, but Different

July 1, 2005 show

They may have been born identical...but today we'll meet some twins who say there has always been something inherently different about each other. Liana says that since the age of 3, she can remember her twin sister, Juanita saying that she wished she were a boy. As Juanita grew up, she never got over her desire to be a man. She hid her feelings for as long as she could from her family because of their strict religious upbringing. However the stress and depression locked inside of her, eventually led her to attempt suicide. Juanita felt she was trapped inside the wrong body and as time wore on, the feeling grew stronger. Today former identical twin sisters Liana and Juanita are now twin sister Liana and brother Juan. They'll share their story with us today. We'll also meet two identical twin sisters who grew up with similar interests, except for one. Dawn says when she was younger she always tried to be like her twin sister, Diana but as she got older she began to realize that she was attracted to other women. When Dawn was 19 she kissed her first girl and knew for sure that she was gay. Dawn and Diane will discuss their differences growing up and how their relationship as twins has grown. Dr. Harvey Kliman is a reproductive biologist from Yale University who will discuss some theories about why some identical twins are copies of each other and why some seem to be mirror-opposites. We'll also meet a pair of twins, Tamara and Adriana who were separated at birth but found each other by accident when they were in college. Despite growing up in separate homes, these twins have amazing similarities in personalities and preferences.

Todays guests:

Liana: Juan's identical twin sister who supports her brother's decision.

Tamara & Adriana: Identical twin sisters were separated at birth and adopted into different families but found each other when they were going to college.

Dawn & Diana: Identical twin sisters who are similar in many ways except for their sexual preferences.

Juan: Born as an identical twin girl named Juanita, who always felt that she was meant to be a boy and is now going through the transgender process.

Harvey Kliman, MD, PhD: a placenta expert who will help explain how some twins are the same, while others are so different.