We spoke to Dr Nandita Palshetkar, Scientific Director & Head of the Bloom IVF Unit at Lilavati Hospital, Mumbai, and Medical Director of Bloom IVF Centres, to understand this better and get you verified information.
How Does One Twin Absorb the Other in the Womb?
In an early twin pregnancy, one embryo may stop developing, often during the first trimester. The pregnancy may then continue with the surviving embryo. When an embryo stops developing, its tissues can be reabsorbed by the mother's body, placenta, or, in some cases, the surviving twin.When cells from the two embryos remain in the surviving baby, those cells can carry different DNA. This creates twin chimerism.
Another mechanism is exchange of cells between twins through a shared placenta. This is more relevant when twins have placental connections that allow blood or cells to pass between them.
A rare type, called tetragametic chimerism, happens when two separately fertilised eggs fuse early in development and form one embryo. The resulting person develops from cells originating from both embryos.
What Is Vanishing Twin Syndrome?
Vanishing twin syndrome refers to the spontaneous loss of one foetus from an initially recognised multiple pregnancy. It is most commonly diagnosed in the first trimester. Many cases cause no noticeable symptoms and are discovered only because an earlier ultrasound showed two gestational sacs or embryos, while a later scan shows one continuing pregnancy.Early ultrasound has made vanishing twin pregnancies easier to identify. Current international ultrasound guidance estimates that a vanishing twin occurs in around one in five twin pregnancies and is more common following assisted reproductive technology.
Chimerism vs Vanishing Twin Syndrome: What Is the Difference?
| Feature | Chimerism | Vanishing twin syndrome |
| Definition | A person carries cells with two different sets of DNA | One embryo stops developing, and its tissue is absorbed |
| What it describes | A lasting genetic state in the person | A pregnancy event |
| When it occurs | Cells are exchanged or merged before birth, and persist for life | Usually in the first trimester |
| Cells from the other twin | Present in blood or other tissues | Usually none remain |
| Common cause | Embryo fusion, or blood exchange through a shared placenta | Chromosome or developmental problems in the embryo |
| How it is found | Blood group mismatch, DNA test, or transplant matching | Early ultrasound showing two sacs, later one |
| Effect on the survivor | Usually none | Usually none |
| Does it need the other condition? | No. It can occur when both twins survive or when two embryos fuse | No. In most cases, no cells remain in the survivor |
Chimerism is not something that should automatically be viewed as a disease or a cause for concern. Many cases may go completely unnoticed and have no impact on the individual’s health.
What Causes Chimerism?
Chimerism has three causes.- Fusion of two embryos (tetragametic chimerism): Two eggs are fertilised by two sperm, forming two embryos. Early in pregnancy, the embryos merge into one. The baby has cells with two different DNA profiles throughout the body. Few cases are documented, and the reason embryos fuse is not known.
- Blood exchange through a shared placenta (blood chimerism): Non-identical twins can share a placenta, and blood vessels connect their circulations. Blood stem cells cross between the twins; each carries some of the other's blood cells. It is found in about 8% of non-identical twins.
- Cells left behind by a lost twin or exchanged with the mother (microchimerism): A small number of cells from a vanished twin, or from the mother, stay in the baby's body and can persist for years.
Signs and Symptoms of a Chimaera
A chimaera is a person whose body contains cells from two genetically different individuals, each with its own DNA. Most chimaeras show no signs. When signs appear, they can include:- Patches of skin with different pigmentation
- Eyes of two different colours
- Two blood groups in a single blood test
- In rare cases, differences in the development of reproductive organs, when the two embryos had different sexes
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FAQs on Chimerism in Twins: When One Twin Absorbs the Other in the Womb
- Can a twin absorb another twin in the womb?
Yes, in vanishing twin syndrome, where a non-viable embryo's tissue is absorbed. Chimerism develops only in some of these cases, when cells from the lost twin remain in the survivor. - Does chimerism need treatment?
No. Most chimaeras are healthy. The condition matters mainly for transplant matching and interpreting DNA test results.