Archive for January, 2010

Twins; How do they develop?

Monday, January 18th, 2010
**Articles written by Cheryl are not intended to be authoritative, but rather to help others in their quest for information on various topics of interest.

The majority of twin births result from the release and subsequent fertilization of two ova (zygotes) by two different sperms. These fraternal twins may be of either sex and are no more genetically alike than ordinary siblings. Scientifically they are termed dizygotic twins, indicating their origin from two zygotes.

Twins births may also originate from the division of a single zygote at some stage during the development of the embryo. Fetal membranes and placenta(s) formed will depend on the stage at which the division occurs. These twins are genetically identical and of the same sex. They are known as monozygotic twins, indicating their origin from one zygote.

Ultrasound is the best modality for early diagnosis of a twin or multiple gestation. At six weeks, if more than one embryo of fetus is identified, each with a visible heartbeat, a diagnosis of a multiple gestation can be made.

Ultrasound also plays an important role in determining the fetal placentation type and any related complications. Twins, compared in singletons, are at increased risk of fetal anomalies, complications and mortality,. Information obtained from ultrasound examinations aids in the management of these high risk pregnancies and can lead to an increased rate of survival.


As with dizygotic twins, monozygotic twins that are dichorionic and diamnoitic, will have separate placentas that may fuse if they implant close to one another. The thick membrane dividing them will consist of four layers, two layers of amnion and two layers of chorion.

Monochorionic, diamniotic twins will share a common placenta and the membrane separting them will be thin, consisting of two layers of amnion.

Monochorionic, monoamniotic twins share a common placenta in which no membrane divides them. The significance of monochorial placentae is its effect on fetal circulation. In approximately 90% of monochorical placentas there is some sort of connection between the fetal circulation of twins.

The placenta is responsible for a number of functions essential to the growth and development of the fetus. One main function of the placenta is respiration. Oxygen from the maternal blood diffuses across the placental membrane into the fetal blood and carbon dioxide from the fetal blood passes in the other direction. Nutrients required by the fetus also diffuse from the maternal to the fetal blood through the membrane. Other functions include excretion of fetal waste, storage of carbohydrates, proteins, calcium and iron, hormonal production and protection.

Twin gestations place an increased demand on placental functions and maternal blood supply and therefore are susceptible to a number of problems. Placentation type plays a significant role in the type of complications that can occur.


Increased risks common to all varieties of twins result from the high frequency of premature delivery and low birth weight. Prematurity, with its complications of immature lung development and cerebral haemorrhage, is an important contributor to the high morality rate of twins. Intrauterine growth retardation adds to this risk

The majority of monozygotic twins are monchorial and frequently encounter additional complications. Whenever twins shared common placenta there is the danger that one twin will recive more of the maternal blood supply than the other. This twin-twin transfusion results from vascular communication between the twins, with one twin growing at the expense of the other. This syndrome is associated with the donor twin at risk from anaemia and growth retardation and the recipient twin at risk from increased cardiac and pulmonary output.

With monozygotic, monoamniotic twins there is often cord entanglement because the twins are not separated by a dividing membrane. These pregnancies most often end prematurely because the entwined umbilical cords cut off the blood supply.

On the rare occasion, when separation of the embryonic disc is incomplete, (in monozygotic, monoamniotic twins) conjoined twins will result. Separation and survival of these twins depends on where they are joined and the organs they share.


Ultrasound is useful in determining the zygosity of twins. This is important as complications and mortality rates vary considerably with placentation type.

Identification of two placentas and a difference in the sex of the twins is indicative of dizygosity.

When a single placenta is seen with a thick membrane separating the twins, dichorionicity can be assumed. Dichorionic twins can be either monozygotic or dizygotic. If the twins are of different sexes they are known to be dizygotic however, if the are of the same sex, zygosity by ultrasound cannot be determined.

A thin membrane suggests monochorionicity and all monochorionic twins are monozygotic.

Monochorionic twins can be either monoamniotic or diamniotic. If a dividing membrane is seen they are diamniotic, but nonvisualization of the membrane does not mean they are monoamniotic, the membrane may be present but not visualized. Entwining of the umbilical cords indicates monoamniocity.

As a sonographer, it is important to have a clear understanding of placentation, fetal membranes and zygosity, with its associated complications, so that proper obstetrical management and informed decisions concerning the pregnancy can be made.

Article written by Cheryl Mech


Beck, F., Moffat, D.C., Lloyd, GB (1973). Human Embryology and Genetics, : Blackwall Scientific Publications.

Benacerraf, B.R. (Ed.) (1990). The Radiologic Clinics of North America. : Fetal Ultrasound (Volume 28 / Number 1). W.A. Sounders Company

Bulmer, M.G. (1970). The Biology of Twinning in Man. Clarendon Press.

Moore, K.L (1998). Essential of Human Embryology. B.C. Decker Inc.

Moore, K.L (1982). The Developing Human. (3rd ed.). W.B. Saunders Company

Potter, E.L., Craig, J.M. (1975). Pathology of the fetus and the Infant. (3rd ed.). Year Book Medical Publishers Inc.

Sanders, R.C. (1991). Clinical Sonography, A Practical Guide. (2nd ed.). Little, Brown and Company

December BabyView of the Month

Friday, January 15th, 2010

Congratulations to our December and our final BabyView of the Month winners Jeannie and David from Oshawa, Ontario

How did you feel about your 3D ultrasound?

My husband and I were in awe at the sight of our first baby. Just to see him in such detail was amazing and exhilarating all at once. We were so focused on his movements and were surprised at the range of motion that he is capable of. Of course he was being a cheeky monkey and was hiding his face from us for most of the ultrasound, but Cheryl stuck with it and we finally got a fantastic shot. We were so happy to confirm we were having a baby boy.

Describe some of your feelings and overall experience at BabyView?

We were amazed when our baby smiled at us, waved to us and grabbed his ankles. He was so wonderful to watch. We felt a range of emotions while we watched him move, Happiness, Love, and Fascination. The atmosphere was so relaxed and it was wonderful that Cheryl was also excited about our baby. She made us feel so comfortable and we felt right at home at BabyView. We came right home and sent the photos to all of our family and friends.

Who are the other family members awaiting your baby’s arrival?

This little boy has been so loved from the moment we found out we were pregnant. His Amma Margaret, Poppa Don, Nana Donna and Grandpa Jim are anxiously awaiting the arrival of their baby. The aunts and uncles as well as our extended family of friends are also very eager to meet him. We cannot wait to introduce our little one to his fur brothers and sisters, two dogs, Emma and Fisher and two cats, Harley and Butters.

Does your baby have a name yet? If so, is he/she named after anyone special?

The name we chose for our little boy is – Liam David Wallace – Liam is a name that we chose together. The name David comes from his Daddy and Wallace is to honor his mommy’s Grandmother, whom is missed everyday, but will be watching over him from Heaven.

Did you think your baby resembled anyone in your family? Why or how?

Liam is a beautiful baby. He has his daddy’s big hands and mommy’s nose. We cannot wait to see what other features he is going to have. He is perfect already, we could never have imagined he would be so handsome.

What are your hopes and dreams for your baby’s future?

We just want our Liam to know how loved he is. He will be surrounded by love and laughter everyday. We hope that he is healthy and strong, and lives a full life surrounded by those who love him. We also want him to be confident in all he does, live life to the fullest everyday, and to appreciate the little moments life has to offer. We would like Liam to have a generous heart, and to share it with the world even if it is in a small way. He has already enriched our lives just by being. We cannot wait to meet our little boy and are excited to welcome him into this wonderful world.

Keep a lookout for our new series called “Ask Cheryl” Where you can ask Cheryl Mech, the owner and registered medical sonographer any questions regarding your sonogram or 3d ultrasound and she will help out with the answers!

See you soon!