Twins in different sac

The Difference Between Identical and Fraternal Twins

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Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.

If twins are a boy and a girl, clearly they are fraternal twins, as they do not have the same DNA. A boy has XY chromosomes and a girl has XX chromosomes. Girl-boy twins occur when one X egg is fertilized with an X sperm, and a Y sperm fertilizes the other X egg.

Sometimes health care professionals identify same-sex twins as fraternal or identical based on ultrasound findings or by examining the membranes at the time of delivery. The best way to determine if twins are identical or fraternal is by examining each child’s DNA. Occasionally a family is told that their twins are fraternal based on placenta findings, when they are in fact identical. Other times, a family may see the minor differences in identical twins and declare the twins fraternal based on these differences in appearance. There are a few commercial laboratories that, for a fee, will send families DNA collection kits to determine if the twins are identical or fraternal. The families swab the insides of each child’s cheek for a DNA sample and send the kit back to the laboratory to await results.

Identical twins have the same DNA; however, they may not look exactly identical to one another because of environmental factors such as womb position and life experiences after being born. Our family joke about one of our twin’s stitches for a lacerated upper lip was that he wanted to distinguish himself from his identical twin brother! In addition to life’s bumps, bruises, and differing hairstyles, a child’s DNA is constantly adapting to that child’s experiences. Different stretches of one’s DNA can turn on or off in response to environmental surroundings—therefore, over time, a pair of identical twins’ DNA becomes more and more distinctive. All twins, whether fraternal or identical, are truly 2 separate, unique individuals.

Author: Shelly Vaziri Flais, MD, FAAP

Last Updated: 11/7/2019

Source: Raising Twins: From Pregnancy to Preschool (Copyright © 2010 American Academy of Pediatrics)

The information contained on this Web site should not be used as a substitute for the medical care and advice of your pediatrician. There may be variations in treatment that your pediatrician may recommend based on individual facts and circumstances.

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Twins & Multiples

A space to give you hints about what is happening in the pregnancy and birth world, it’s common problems, and how to solve them

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Twins in separate sacs. You might have heard your doctor or another medical professional talking about this, or maybe they mentioned it to you directly, and you’re wondering what it means.

What Does It Mean When Twins Are in Separate Sacs?

No matter how you heard about it, relax. It’s nothing to worry about, because it’s mostly just medical jargon.

Twins in two separate amniotic sacs is normal. It’s the most regular type of twins, the most regularly occuring, and it comes with the least amount of side effects and possible problems, so you should feel good! This is a good sign for you and your babies!

There are actually two types of situation where your twins are in separate sacs, called dichorionic diamniotic twins, where your twins have separate sacs and placenta, and monochorionic diamniotic twins, where your twins share a placenta, but have separate amniotic sacs.

We’ll go into the differences below.

Di Di: Twins in Separate Sacs with Separate Placentas

This is the most common type, and like we’ve just explained, it’s the one that comes with the least amount of side effects an unfortunate problems. So that’s good!

Di di twins can be identical or non-identical, but are actually more likely to be non-identical, because of the way this type of twin occurs in your womb.

Now, every pregnancy is hard, but this one, where your twins only share a living space and nothing else, is by far the easiest for both you and your babies.

There’s very few complications that can happen with these types of twins. The most likely thing is the slightly premature birth and low birth weight that’s common with almost all twins, so try to relax.

Mono Di: Twins with One Placenta and Two Sacs

Mono Di twins, or twins that share a placenta whilst having their own amniotic sacs, start as just one egg which splits a few days in, for reasons that are still unknown!

Because they come from the same egg, this type of twin is always identical, so if you really want a pair of identical twins and you hear that your twins share a placenta, you’re getting your wish!

Unfortunately, because this type is a little atypical, there’s a few more problems that can occur.

Luckily, advances in modern medicine and surgical procedures have made these much less likely to cause long term problems, so your chances of going to full term and delivering a pair of healthy identical twins is the highest it’s ever been!

If your twins share a placenta, your doctor might want to bring you in for more regular scans, and other things that friends and family that have gone through pregnancies won’t have had. Don’t worry. This is normal, and they’re experienced in dealing with women in your situation.

To Recap, What Does It Mean When Your Twins Are in Separate Sacs?

Your twins are probably going to be in separate amniotic sacs, this is completely normal.
Your twins being in separate sacs means that your pregnancy is more likely to be safe and problem free.
There are two types of twin situations where your babies can be in different sacs, separated by whether they share a placenta, (mono di) or have separate placenta (di di. )
Di di twins are the most common type, don’t have to be identical, and is most likely to be the easiest.
Mono di twins share a placenta, and so you are a little more likely to have issues, but modern medicine means that as long as these issues are found early, you and your babies are going to be okay.
Whether you’re mono mono, or mono di, you shouldn’t worry.

Remember, the best thing you can do is be informed, so we’re glad you’re here. At Twinstuff, we’ve got articles on everything you might face during your pregnancy, so take a look around.

We’ve also got our Facebook group where you can meet other parents. And if you’ve got any questions about twins in separate sacs feel free to ask us in the comments below! We love to talk to moms and expectant moms to be, just like you!

Rare semi-identical twins born in Australia

February 28, 2019

Photo copyright, Queensland University of Technology

Image caption,

If one egg is fertilized by two sperm cells, a triploid can be formed that has three sets of chromosomes instead of the usual two.

A boy and girl born four years ago in Australia are identical twins on their mother's side. This case is reported in The New England Journal of Medicine.

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However, they share only part of their father's DNA, which, from the point of view of genetics, makes them intermediate between monozygotic (identical) and dizygotic (non-identical) twins.

According to experts, this type is extremely rare - usually such embryos do not survive.

Professor Nicholas Fisk, who led a team that looked after the mother and twins at the Royal Women's Hospital Brisbane in Australia in 2014, said the discovery was made during a routine pregnancy scan.

These twins were first discovered before birth. Their mother, who was 28 years old, had not previously had a pregnancy and conceived naturally.

"An ultrasound of the mother after six weeks of pregnancy showed a single placenta and a characteristic arrangement of the amniotic sacs, which indicated the presence of identical twins," says Prof. Fisk.

"However, an ultrasound performed after 14 weeks of pregnancy revealed that the twins were a boy and a girl, which is impossible with identity."

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Monozygotic (also called identical or homozygous) identical twins are formed from one zygote (one egg fertilized by one sperm), divided into two or more parts during the crushing stage. They have the same genotypes.

Monozygotic identical twins are always of the same sex and have a very large portrait resemblance. Among monozygotic twins, there is often a great similarity in characters, habits, and even biographies.

Heterozygous (non-identical) twins develop when two eggs are fertilized by two sperm. Heterozygous twins have different genotypes. They are no more similar to each other than brothers and sisters, since they have about 50% of identical genes. Interestingly, in rare cases, heterozygous twins can be born from different fathers. Sometimes heterozygous twins share a fused placenta.

In this case of semi-identical twins, the egg was probably fertilized by two sperm at the same time before it split.

If one egg is fertilized by two sperm cells, a triploid is formed instead of the usual diploid, having three sets of chromosomes instead of the usual two - one from the mother and two from the father.

According to scientists, three sets of chromosomes are incompatible with life and such embryos usually die in the early stages of development.

Semi-identical twins are not named.

Why are twins born?

Image copyright Getty Images

Non-identical twins are more common in families that have had twins before. They appear more often in late-born mothers, since they often have several eggs at the same time during the process of ovulation.

The appearance of identical twins is not related to whether they were in the family of the spouses. Fertility treatments can increase the chance of having twins.

About 12,000 twins are born in Britain every year.

"Exceptional case"

The first documented case of the birth of semi-identical twins was noted in the USA in 2007.

Professor Fisk notes that an analysis of the statistics available in the world confirms the rarity of this phenomenon.

He and his colleagues examined genetic data from 968 fraternal twins, but could not find any such case.

"This is an exceptionally rare case," says the professor. "While it is useful for physicians to have it in mind when giving birth to identical twins, its rarity means that it is not worth introducing a special genetic test for it."

Monoamniotic twins (sharing one amniotic sac): would it be better if they were born earlier?

What is the problem?

Identical twins who share the same amniotic sac in the uterus are called monoamniotic twins. The amniotic sac is the bag of water (amniotic fluid) surrounding the baby. Monoamniotic twins are different from most twins, who are separated from each other by their own amniotic sacs and their own amniotic fluid.

Monoamniotic twins are very rare, with a pregnancy rate of 1 in every 10,000 or about 1% of all identical twin pregnancies. Pregnancy with monoamniotic twins has its own increased risks - in addition to the risks that all types of twin pregnancies have - they include the death of one or both twins. The increased risks are explained by the fact that the umbilical cord can be clamped and the oxygen supply to the baby can be blocked.

We wanted to find out if it would be better to deliver early birth of monoamniotic twins by induction or caesarean section (planned preterm birth) before 34 weeks of gestation or , wait until 36-38 weeks (or until labor begins on its own), while maintaining strict control over pregnancy and twins ("expectant management").

Why is this important?

Pregnancy with monoamniotic twins carries higher risks than other types of twin pregnancies, and these risks include the death of one or both twins. It is possible that the early birth of twins - before 34 weeks - reduces the risk of cord compression. However, the consequences of preterm birth can also be serious, and the need for hospital care for some time is almost impossible to rule out.

What evidence did we find?

We searched for randomized controlled trials that compared planned early birth with expectant management (search date 31 March 2015).