Can twins that develop in separate sacs but share the same placenta be identical?

I have two younger non identical twin sisters. When my mum was pregnant she was told they were both in different sacs when developing but shared the same placenta.

We have done hours of research on the topic, as we have reason to believe they could after all actually be identical.

After all this research we've just found a load of fancy names for twins and no information on non identical twins that share the same placenta and are in different sacs.

Another thing that may help is that they are mirror twins.

they were both in different sacs when developing but shared the same placenta

This suggests that they were monochorionic diamniotic twins, which would mean they are monozygotic, i.e. identical.

Twin Pregnancy Complications

A single placenta normally supports a single fetus. When the situation arises in which two fetuses have to share a single placenta, complications may sometimes develop. Identical twins that share a single placenta are called monochorionic twins (MC). “Chorion” is the Latin root that refers to the placenta, while the word “amnion” refers to the sac, or “membranes” that surround each fetus. While fraternal twins (2 eggs and 2 sperm) are always surrounded in their own sacs and have their own individual placentas, 70% of identical twins may end up sharing a single placenta. Only 1% of identical twins share both a single placenta and a single sac, and this poses significant risk.

When two fetuses share one placenta, their umbilical cords may implant anywhere – there is no set or predictable pattern – and depending on where they implant, one fetus may get less of a ‘share’ of the placenta than it’s co-twin, resulting in less blood flow and nutrition to one fetus, with more to the other (unequal placental sharing). As a result, although identical twins usually share the same genetic material, they may actually grow differently. Like the roots of a tree, the blood vessels that run from each implanted cord may connect with each other beneath the surface, as there is nothing separating them within a single placenta. Depending on which types of vessels connect to which, one fetus may transfuse blood to the other. We will discuss each of these complications, their risks, and potential treatments, below.

Identical twins

Sometimes a fertilised egg splits within a few days of conception to produce genetically identical twins. Because these twins come from one zygote, they are also known as monozygotic. Identical twins are the same sex.

There are three types of identical twins.

About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas.

The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic.

In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac, called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins. They&rsquore often called MoMo twins.

Identical twins happen in around 1 in 250 pregnancies in Australia.

Although identical twins have the same genes, they don&rsquot always look the same. This is because children&rsquos health and development are shaped not only by genes but also by experiences in the womb and after birth. For example, a twin who receives less blood from a shared placenta might weigh less at birth.

Can you have two placentas?

In the days and weeks that follow conception, the swiftly multiplying cells become your baby and placenta. If you are carrying one baby (singleton pregnancy), then only one placenta will develop.

It’s possible for more than one placenta to form – for example, if you are pregnant with twins or triplets. This is the most common reason to have two placentas in pregnancy.

Multiple pregnancies are becoming more common due to the increase in fertility treatments and because women are having babies later in life.

According to the NHS, one in every 65 births in the UK will be twins, triplets or more. The Centers for Disease Control and Prevention states twin births have nearly doubled over the last 40 years in the US, and triplets and higher order births have quadrupled.

#4: What makes identical twins identical?

Prepare to have your mind blown: identical twins are not always identical!

How is it possible that people formed from the same cells aren’t exactly the same?

We’re not just talking about being able to tell twins apart from their outward appearance. While these twins share their looks, twins that are identical can also have different personalities and different susceptibility to disease. This takes us back to the age-old question of ‘nature versus nurture’ and the roles both play in development.

As researchers explain, exposure to different environments can cause changes and adaptations in human gene expression.

But geneticist Carl Bruder and his team went deeper, and found some differences occurred even at the genetic level in monozygotic twins.

Bruder, in Scientific American (2008) says, “I believe that the genome that you’re born with is not the genome that you die with—at least not for all the cells in your body”.

So what does it all mean?

Basically, what all of this science lingo regarding the biology of twins means is that unless your twins are MoMo, MoDi, or conjoined you won&rsquot know for sure unless you have genetic testing done.

If you&rsquore like me, you might have your twins and assume they are fraternal. You might field tons of questions about why they look so alike if they aren&rsquot identical. And then you might find out 13 years later that you were wrong. They actually are identical and your vision has been blurred by mom-goggles. Try not to be too embarrassed. It happens.

If you just heard that second heartbeat for the first time, or you know it&rsquos been two for a while, you need to read our twin pregnancy week by week timeline to help you learn what happens week by week with twins. Click here to learn more&hellip and while you&rsquore at it, check out our expecting twins classes and twin parent coaching services.

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 diamnionic twin placenta

Conjoined twins

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

In the Womb: Twin Pregnancy

Mothers will not immediately tell if they have conceived twins. Suspicions may arise because of the severity of pregnancy symptoms. An ultrasound can easily determine whether the expecting mom is having twins.

The Good

In multiple or twin pregnancies, babies are already starting to bond inside the womb. Starting at around 14 weeks, twins start to reach out to each other and bond.

They already learn how to share an amniotic sac, placenta, and nourishment from their mother. These factors contribute to their physical and emotional well-being when they are out of the womb.

The Bad

In terms of pregnancy symptoms, expectant mothers may experience more severe symptoms and the pregnancy will be considered as high-risk. Upon discovery of the multiple pregnancy, they will be closely monitored by their doctors because of the babies’ increased risk for complications and preterm labor.

The Worst Case Scenario

Multiple pregnancies do not only pose a risk to unborn babies, but to the mother as well. In worst cases, even if the pregnancy is closely monitored, complications still happen and cause the fetal death of one or both babies.

With proper monitoring, diet, and prenatal care, multiple pregnancies can be managed, and both twins and mother survive all associated problems and complications.


Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), but sometimes, in the case of monoamniotic twins (Monochorionic-Monoamniotic "MoMo"), they also share the same amniotic sac. Monoamniotic twins occur when the split takes place after the ninth day after fertilization. [2] Monoamniotic twins are always monozygotic (identical twins). [3] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused. [2]

By performing an obstetric ultrasound at a gestational age of 10–14 weeks, monochorionic-diamniotic twins are discerned from dichorionic twins. The presence of a "T-sign" at the inter-twin membrane-placental junction is indicative of monochorionic-diamniotic twins (that is, the junction between the inter-twin membrane and the external rim forms a right angle), whereas dichorionic twins present with a "lambda (λ) sign" (that is, the chorion forms a wedge-shaped protrusion into the inter-twin space, creating a rather curved junction). [4] The "lambda sign" is also called the "twin peak sign". At ultrasound at a gestational age of 16–20 weeks, the "lambda sign" is indicative of dichorionicity but its absence does not exclude it. [5]

In contrast, the placentas may be overlapping for dichorionic twins, making it hard to distinguish them, making it difficult to discern mono- or dichorionic twins on solely the appearance of the placentas on ultrasound.

In addition to a shared placenta, monochorionic twins also have their circulatory systems intermingled in random and unpredictable circulatory anastomoses. This can cause disproportionate blood supply, resulting in twin-to-twin transfusion syndrome (TTTS) in 20% [1] of MoDi pregnancies. This is the main complication of monochorionic twins.

The 80% of MoDi pregnancies without TTTS still have high rates of birth weight discordance, fetal growth restriction, prematurity and resultant cesarean section deliveries. [1] One twin may also fail to develop a proper heart and become dependent on the pumping activity of the other twin's heart, resulting in twin reversed arterial perfusion. [2] If one twin dies in utero, blood accumulates in that twin's body, causing exsanguination of the remaining twin. [2]

In the case of monoamniotic twins the risk of complications is substantially higher because of additional potential umbilical cord entanglement and compression. [3] However, the perinatal mortality of monochorionic twins is fairly low. [1]

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Megan Loden, COO: &ldquoChief of Other&rdquo Stuff No One Wants to Take Care Of. Associate Editor. Mentor Program Coordinator. She is Mom to 17-year-old identical twins girls and a 12-year-old son. When she isn&rsquot busy embarrassing her twins with her mere presence, she can be found obsessing over her 12-year-old son or talking to her dogs and cats while her husband answers on their behalf, voices and all.