Fraternal twins that are not identical are the most common type of twins. They are also known as dizygotic twins. What this means is that each twin forms from an individual fertilized egg. By the third week of pregnancy, each fertilized egg forms into a ball of hundreds of cells and becomes known as a blastocyst. At this time, the newly formed embryo burrows into the lining of the uterus to be nourished and develop into babies.

Identical twins are a rare form of twin and develop from the same fertilized egg that splits into two babies. They will go on in pregnancy to share the same placenta and look exactly alike after they are born.

Important notes: Every pregnancy is different and this article about "twin pregnancy week by week" is only a rough idea of fetal development.

Twin Pregnancy Week by Week: First Trimester







Fraternal Twins: Two fertilized eggs (zygotes) burrow in the lining of the uterus. They each have their own placenta and umbilical cord. They can be two males, two females or one of each.

Identical Twins: One egg has split into two babies and they are always either two girls or two boys. They have burrowed into the lining of the uterus and share the same placenta, same amniotic sac, but each has their own umbilical cord.

4 Weeks


The Nervous System: At this stage of pregnancy, your baby’s neural tube is forming. This will turn into the brain, spinal cord and nervous system.

The Circulatory System: Tiny blood vessels are beginning to form and beginning to circulate nutrients between you and your baby. Their tiny hearts are already beating! All the major organs are beginning to form including the intestines and the tiny buds which will develop into arms and legs. They are about the size of small seeds at this stage and they have tiny depressions where the mouth, nose and ears will soon appear.

8 Weeks



Organ Development: Twins’ hearts are beating at about 150 beats per minute and have two chambers already developed. The teeth, ears, brains and liver have formed and are continuing to develop into sophisticated organs. On the outside, their skin is very translucent and can be seen through. The tiny toes and fingers are beginning to separate. At this time, their eyelids are forming over the eyes.

12 Weeks


Organ Development: The livers are beginning to function and their kidneys now produce urine. The babies take in the amniotic fluid by mouth and the kidneys then change it to urine. This is actually a healthy process that helps with development. Because their brains continue to form and develop, the nervous system can now produce reflexes in the muscles, but you may not feel any kicks yet. The eyes are moving to the front of their faces and the ears are moving to the sides.

Size: Your twins are getting big! They are about 2.1 inches long.

Twin Pregnancy Week by Week: Second Trimester



16 Weeks


Size: This is a very rapid period of development. Your twins are almost 4 inches long. As a matter of fact, their bodies are growing faster than their heads and their legs growing faster than their arms. These little ones now only weigh 100 grams or two-tenths of one pound.

Organ Development: Their lungs begin to move in and out as if they are breathing. Female twins begin to produce eggs in their ovaries. At this time the spinal cord becomes covered with the myelin sheath to protect it. The bones are very soft and rubbery (cartilage). They can bend at the joints and the fingernails are forming at this time.

20 Weeks


The Nervous System: This is a big time for your twin’s nervous system. Nerve fibers are beginning to connect and they are developing their senses of smell, taste, touch, hearing and vision. Your babies will know your smell right at birth.

Size: Your babies are now about 10 inches long each from head to toe. With twins, it starts getting a little crowded because they have grown so much and you may begin to feel lots of pushes and shoves.

Organ Development: During this time, your babies will have a very thick coating to protect their skin called vernix caseosa. This protects them from this long time immersed in the amniotic fluid and can also protect them from infections that can be passed at birth.

24 Weeks


Organ Development: Even though your babies do not have an eye color yet, their eyes are fully formed now. They even have tiny little eyelids and eyebrows. The endocrine system is now starting to mature and getting ready to produce hormones that will regulate their body functions. Their lungs are still developing and can move in and out developing the breathing muscles. However, babies born at this age usually cannot breathe on their own.

Twin Pregnancy Week by Week: Third Trimester



28 Weeks


Size: Twins at this stage already weight about 2 pounds each and are about 15 inches long. They are beginning to deposit layers of fat and their bones are beginning to harden.

Organ Development: Your twins are developing large amounts of brain tissue during this period. Their hearing is becoming more tuned and they can hear your voice. They can also see bright lights that shine on your belly and will turn their heads toward the light. Sexual organs develop. Boys testicles descend and girls develop their clitoris.

32 Weeks


Size: Twins during this time period begin to look more like newborns. They are about 16 inches long and weigh about 3.7 pounds. Twins tend to settle down at this time because there isn’t much room to move around.

Organ Development: Your twins may or may not have hair yet. Their kidneys are producing urine as they take in amniotic fluid and urinate. Their skin is filling out with fat and they are losing their wrinkled appearance. All of the bones in their body continue to become solid, except for their skull bones that remain soft to go through the birth canal.

36 Weeks


Size: Twins on average tend to be slightly smaller than single full-term infants, but not always. At this stage, twins grow to about 5.5 pounds, but can be up to 8.0 pounds. They are about 20 inches long, more or less at birth.

Organ Development: The skin is beginning to shed the covering (vernix caseosa) and prepares for the outside world. The organs are now mature and ready for the babies to be born, with the exception of the lungs which need this final four weeks to finish maturing. In some cases, twins are born early so doctors have treatments to help the lungs mature faster.