Term 3 Science ACEFormation of twinsUpdated 2012Copyright Ng Wei Kai (2i1 16)

Fertilisation Before we learn more about twin conception, let

us first understand how single babies are formed via fertilisation.

A single ovum is released once every month from a woman’s ovaries during the ovulation stage of the menstrual cycle.

Fertilsation The ovum then travels to the uterus via the

fallopian tube. During this period, sperms from males swim up

the fallopian tube to mate with the ovum. The male and female gametes fuse to form a

zygote, restoring the diploid number of chromosomes

Fertilisation After fertilisation, the zygote goes through

mitotic cell division known as cleavage. Many cells are then formed from the cytoplasm

of the zygote This ball of cells is called a blastocyst The blastocyst then embeds itself in the uterine

lining

Fertilisation Upon implantation, the developing baby is called

an embryo. After 9 months of development, it will become a

fetus. During this period, it is protected by the

amniotic sac, which is filled with amniotic fluid. Supplied nourishment and oxygen and removed

of waste via the umbilical chord and placenta.

Twin conception Now that we understand how single babies are

formed, let us move on to the conception of twins.

Two types of twins:1. Identical (monozygotic)

2. Fraternal (dizygotic)

Fraternal twins Two-thirds of all twins are dizygotic occur when two sperm fertilize two distinct

eggs. In this instance, rather than releasing one egg during ovulation, the mother has released two.

Fraternal twins Fertilisation of two separate ovum results in two

distinct embryos. However, if these embryos implant very near to

each other along the endometrium, the two placentas can become fused.

Genetically similar, not identical

Multiples births Multiple births can also be carried out using this

process if more eggs are released For example, if three eggs are released and

fertilsed, the children born would be known as trizygotic triplets.

Interesting fact! Nadya Suleman from Bellflower, California gave

birth to 6 males and 2 females, who were conceived via In-vitro fertilisation, on January 26, 2009

They are fraternal since they have different genders.

Identical twins Occurs when one sperm fertilises one egg Instead of having the blastocyst remaining as

one cell group, it splits in two. The two balls of cells then develop into two

different embryos.

Identical twins Since the twins come from the same fertilsed

egg, they will share identical features such as same gender.

Bibliography [Untitled image of fraternal twins]. Retrieved 17 August 2012, from : http://

i.dailymail.co.uk/i/pix/2009/09/26/article-1216253-069872FB000005DC-530_468x317.jpg

[Untitled image of gestation] . Retrieved 17 August 2012, from : http://static-www.icr.org/i/articles/af/made_in_his_image_gestation_wide.jpg

[Untitled image of identical twins]. Retrieved 17 August 2012, from: http://www.mombaby.org/UserFiles/File/TTTS_images/TTTS01.jpg

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http://spilldabeanz.com/wp-content/uploads/2012/06/octomom6.jpg

[Untitled image of blastocyst]. Retrieved 17 August 2012, from: http://4menandalittlebaby.files.wordpress.com/2011/03/blastocyst.jpg

Twins. (n.d.). Retrieved 15 August, 2012 from wikipedia: http://en.wikipedia.org/wiki/Twin

Neer, K. (n.d.). Howstuffworks.com. Retrieved from http://science.howstuffworks.com/environmental/life/genetic/twin.htm