15.5 Formation of Twins
Twins refer to two or more children that are born from a single pregnancy. Depending on how fertilisation and embryonic division take place, twins are categorised into two main types: identical twins and fraternal twins. In rare cases where the embryo of identical twins does not divide completely, conjoined twins develop.
Learning Objectives
- Define twins and identify the two types of twins.
- Explain the process of formation of identical twins and fraternal twins.
- Compare and contrast the characteristics of identical twins and fraternal twins.
- Describe the development, physical characteristics, and challenges associated with conjoined twins.
Comparison of Twin Formation and Conjoined Twins
A clear visual comparison between the fertilisation and developmental pathways of identical twins, fraternal twins, and conjoined twins.
Identical Twins
One sperm fertilises one ovum to form one zygote. The single embryo divides into two separate embryos. Both foetuses share one placenta, have similar genetic makeup and physical appearances, and are always the same sex.
Fraternal Twins
Two sperms fertilise two separate ova independently, forming two separate zygotes. The embryos do not divide; each foetus has its own placenta. Genetic makeup and physical appearances are different, and the sex may be the same or different.
Conjoined Twins
Develop when there is an incomplete division of the embryo in identical twins. Two foetuses are formed, but some physical parts remain fused, commonly at the chest, abdomen or buttocks. They may share one or two internal organs.
Short Explanation
Definition
Twins — two or more children that are born from one pregnancy. There are two main types: identical twins and fraternal twins.
Formation of Identical Twins
One sperm fertilises one ovum, forming one zygote. The single embryo divides into two separate embryos. Both developing foetuses share one placenta. Their genetic makeup and physical appearances are similar because they originate from the same single zygote. The sex of both twins is always the same.
Formation of Fraternal Twins
Two sperms fertilise two separate ova independently, forming two separate zygotes. The embryos do not divide; two separate embryos develop independently from the start. Each foetus possesses its own separate placenta. Their genetic makeup and physical appearances are different because they originate from two different zygotes. The sex of the twins may be the same or different.
Development of Conjoined Twins
Conjoined twins develop when there is an incomplete division of the embryo in identical twins. Two foetuses are formed, but some of their physical parts remain fused together. Common sites of fusion include the chest, abdomen, or buttocks. They may also share one or two internal organs.
Survival and Surgical Separation
Most conjoined twins die before birth or have very short lifespans. Some can be separated through surgery. The success of surgical separation depends on which body parts are joined and which internal organs are shared.
| Characteristic | Identical Twins | Fraternal Twins |
|---|---|---|
| Origin / Fertilisation | Product of fertilisation of one ovum and one sperm forming one zygote | Product of fertilisation of two sperms and two ova forming two zygotes |
| Division of embryo | Embryo divides into two | Embryo does not divide into two |
| Placenta | One placenta is shared between two foetuses | Each foetus has its own placenta |
| Genetic makeup and appearances | Similar genetic makeup and physical appearances (from the same zygote) | Different genetic makeup and physical appearances (from two different zygotes) |
| Sex | The sex of both twins is the same | The sex of both twins may be the same or different |
Try to Answer First
Answer in your mind, then press “Check Answer”.
State two differences between identical twins and fraternal twins based on their placenta and sex.
How do conjoined twins develop?
What determines the success of surgical separation in conjoined twins?
Common Mistakes
- Assuming fraternal twins must always be of different sexes; fraternal twins can be of the same sex or different sexes because they arise from two independent fertilisation events.
- Believing identical twins can share separate placentas or have different sexes; identical twins come from one zygote, share one placenta, and always share the same sex.
- Assuming conjoined twins originate from fraternal twin fertilisation (two ova and two sperms); conjoined twins only arise from identical twins due to incomplete division of a single embryo.
Concept Misunderstandings
Fraternal twins are identical if they look very similar.
Fraternal twins arise from two different zygotes (two separate ova fertilised by two separate sperms); their genetic makeup is different, just like ordinary siblings born at different times.
Conjoined twins are caused by two embryos fusing together during development.
Conjoined twins result from the incomplete division of a single embryo in identical twins, not the fusion of two separate embryos.
Summary
- Twins: Two or more children born from one pregnancy.
- Identical twins: 1 ovum + 1 sperm → 1 zygote → embryo divides into two → share 1 placenta → identical genetic makeup → same sex.
- Fraternal twins: 2 ova + 2 sperms → 2 zygotes → embryo does not divide → separate placentas → different genetic makeup → same or different sex.
- Conjoined twins: Incomplete division of the embryo in identical twins → physically fused at chest, abdomen, or buttocks → may share internal organs → surgery depends on joined parts and shared organs.
Short Activity
Objective: test your understanding of twin formation, differences between twin types, and conjoined twins.
A. Objective Quiz
Which of the following statements is correct regarding the formation of identical twins?
What is the primary cause of the development of conjoined twins?
B. Fill in the Blanks
In fraternal twins, each foetus has its own separate to obtain nutrients from the mother.
Conjoined twins develop when there is an incomplete division of the embryo in twins.
C. Matching / Drag and Drop
Match each characteristic to the correct type of twin. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Characteristic to the Type of Twin
Write the correct type of twin next to each characteristic.
| Characteristic | Type of Twin |
|---|---|
| 1. Embryo divides into two and foetuses share one placenta | |
| 2. Formed from two separate zygotes and have separate placentas | |
| 3. Sex of both twins is always the same | |
| 4. Physical parts remain fused due to incomplete embryonic division |
Answer choices: Identical twins • Fraternal twins • Conjoined twins
