Twins: A Closer Look at Your Developing Babies

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.

Fetal size: Length, 1.61 inches, crown to rump. Weight, 0.25 ounces.

Fetal development milestones: Lots of little arms and legs are growing and elongating this week! Facial features are developing, as are fingers and toes.

What you're seeing: Just 11 weeks into your pregnancy, nine from conception, your unborn babies' faces are becoming more recognizable, with ears and eyes gradually moving into the proper place. Where there were once only small buds, arms and legs are now growing. Each baby has her own amniotic sac and umbilical cord. In this image, the sonographer has marked each twin, one with an A, the other with a B. The baby marked A is always the one closest to the cervix. Here, you can see the cervix in the lower right-hand corner of the image as the grayish form that comes near baby A. This baby has her head facing the cervix (notice the white facial bones), while baby B has her back toward the membrane that separates the two babies.

Fetal size: Length, almost 3 inches, crown to rump. Weight, almost 1 ounce.

Fetal development milestones: Kidney and urinary tracts are now functioning. Your babies can suck on their thumbs. Genitalia are developing too.

What you're seeing: In this 3-D image of your growing babies, you can see how each baby is developing much like he would in a single pregnancy. Notice that the babies' heads are more prominent than their growing limbs. Soon their arms and legs will be in proportion to the rest of their bodies. With just the right angle, the sonographer may be able to view each baby's gender. But at this early point in the pregnancy, the babies' sex will still be difficult to determine.

Fetal size: Length, 3 1/4 to 4 inches, crown to rump. Weight, around 1 ounce.

Fetal development milestones: Your babies' male or female genitalia are forming. Tissue under their faces and limbs is beginning to harden into bone. Their livers and spleens are producing red blood cells. Upper limbs are becoming proportionate to the rest of their bodies (although lower limbs are still underdeveloped).

What you're seeing: Now into your second trimester, your babies' bones are beginning to harden in a complex process called ossification, where inorganic and organic compounds come together to help the skeleton harden. If you look closely at each baby's head you can see the ossified white outlines of the bones in the skull. Baby A has his back toward the right side of the image, so his face is toward the membrane. His twin, baby B, has his back to the membrane (notice the white line of balls indicating the spinal column), his arm to his side.

Fetal size: Length, 5 to 5 1/2 inches, crown to rump. Weight, 5 1/4 ounces.

Fetal development milestones: The babies' senses are beginning to function. Facial features are now in their proper position. Fat tissue is developing. And you may be feeling fetal movement!

What you're seeing: Almost halfway into your pregnancy, your babies' eyes can detect light and their ears pick up on some sounds. Your unborn babies' facial features are continuing to become more refined. In this image, baby B has her back to the right side of the picture. Notice the white grouping, which indicates her spinal column. The dark circular form is her heart, and the faint, bubble-looking form above her is the umbilical cord. Baby A's umbilical cord is also visible at the top of the baby's body. The membrane that separates the two growing babies can move and sway as the fluid in the uterus moves -- or as the babies wiggle. Here, it's swaying toward baby B.

Fetal size: Length, 5 1/4 to 6 inches, crown to rump. Weight, 7 ounces.

Fetal development milestones: Arms and legs are now well-developed and in proportion. The babies' skin is now covered with cheeselike vernix as protection. Fat is beginning to build up, especially around sensitive areas like the neck, sternum, and kidneys.

What you're seeing: It's nearly impossible for the sonographer to capture a picture of both your babies' body profiles. In this image you can see that one baby, BB, is positioned with his head nudging the membrane that separates the two. The other baby, marked here as AA, is looking up and to the left of the image. You may begin to feel all this movement and heads shifting position. While your babies' arms and legs are now well-developed, their movements will still feel like little more than a flutter. In coming weeks, their kicks and jabs will become much more recognizable -- and forceful!

Fetal size: Length, 5 2/3 to 6 1/2 inches, crown to rump. Weight, 9 ounces.

Fetal development milestones: Hair and nails are continuing to grow. In girls, the uterus is now formed and the vaginal canal is forming. You should be able to really feel movement now!

What you're seeing: As your babies grow larger, there's less room for them to maneuver in the womb. In this image the babies look as though their heads are touching. Often the babies' heads will rub together or touch, separated only by the thin membrane that lies between the two. With this image the sonographer is confirming that indeed there are two babies developing in the womb and also that the babies' heads are about the same size. If one head is significantly larger than the other, the sonographer may make additional measurements to ensure that both babies are developing properly. Different fetal head sizes may indicate a potential problem in the pregnancy.

Fetal size: Length, 9 1/4 inches, crown to rump. Weight, nearly 2 pounds.

Fetal development milestones: Fingernails are fully developed. Eyelashes are forming and scalp hair continues to grow.

What you're seeing: In this image you can see how the unborn babies' heads are touching in utero. The hand pointers that the sonographer has placed on the picture point out the babies' hair. At this point in development, unborn babies have been growing hair on their scalps for some time. While not every baby has hair on her head at birth, these two will have some curly locks!

Fetal size: Length, 9 2/3 inches, crown to rump; total length about 15 1/4 inches. Weight, 2 pounds.

Fetal development milestones: Lungs are continuing to develop, and your babies' livers are maturing. Their immune systems are also strengthening.

What you're seeing: Triplets! Marked with A, B, and C, this image captures three babies' heads in one image. It's very unusual for wiggly babies in utero to come together so that a sonographer can fit all three into the view screen. Looking at each head, you can see the bright white outline of the ossified, or hardening, skull bones. As with marking twins in utero, the sonographer tags the baby closest to the cervix with an A, the next closest, B. That will also be the order in which the babies are born.

Fetal size: Length, 10 inches, crown to rump; total length 15 3/4 inches. Weight, almost 2 1/2 pounds.

Fetal development milestones: Added brain tissue is developing, and your babies' scalp hair continues to grow. Your babies are gaining weight and opening their eyes.

What you're seeing: At 28 weeks' gestation, your babies are beginning to get scrunched in their home inside your womb. Although you can't see the membrane in this image, a thin wall separates the two babies. Here, the bright white lines represent ossified, or hardened, facial bones in each baby's head. You may notice that your babies' movements become more sluggish as they have less wiggle room.

Fetal size: Length, 13 1/2 inches crown to rump; total length 20 1/4 inches. Weight, 6 pounds.

Fetal development milestones: A big, healthy weight gain! All body systems are now developed and functioning.

What you're seeing: Only a few more weeks until delivery day! With a twin pregnancy, you're most likely to receive several ultrasounds to confirm that your babies are developing on schedule. As you near your babies' due date, your health care provider may request an ultrasound examination to check the babies' position in the uterus. Here, the two babies are lying cheek to cheek with only a thin membrane separating them.

All ultrasound images for this slideshow were provided by the sonographers of the Johns Hopkins Maternal-Fetal Diagnosis and Treatment Center. We are grateful to Christine Bird, BS, RDMS, RVT, chief obstetrical sonographer, and Jude Crino, M.D., medical director, for their assistance with this project.

For examples of prenatal ultrasounds and more information on your baby's fetal development, be sure to visit www.aium.org and www.marchofdimes.org.

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