“The more, the merrier!” is a phrase that resonates beyond hospital walls moments after delivery of the magical biological phenomenon - twins. In approximately every 80 pregnancies, one is positively estimated to produce two offsprings. Some women are simply more genetically prone to releasing more than one egg per cycle than the rest. If the woman is in her late thirties, chances of twin pregnancies are higher. In-vitro fertilization involves transfer of several eggs to ensure the development of at least one embryo. In effect; the result is quite often- two!
Twins originate from a variety of organic instances. Some newborns may be identical while some aren’t. In some scenarios, twins share the same sex and in others, they don’t. Nonetheless, despite sharing the same birth date and DNA, they are poles apart in terms of personality traits and preferences. This statement can be reinforced by the quote- ‘I may be a twin but I'm one of a kind’. In this piece, you will uncover the various kinds of twins present in this world.
When a single zygote fuses with a sperm and splits into two separate embryos, the final product of this fertilization is what we scientifically call monozygotic twins. They are consistently identical to the human eye, sharing the same physical features due to matching genetic information. Since they stem from the same zygote containing either a male (XY) or female (XX) sex chromosome, their genders can never differ. However, in a few out of the ordinary cases of genetic mutation, a male foetus loses the Y chromosome and develops as a female with a deterred growth of ovaries. One-third of all twins are reported to be monozygotic.
This occurrence is set off by the release of more than one egg in an ovulation cycle. When two separate eggs are fertilized by two separate sperms, fraternal twins are nine months away from earth! Also known as dizygotic twins, separate placentas, chorions and amnions confirm their type in the womb. Around two-thirds of all the twins in this world are fraternal.
Conjoined twins or Siamese twins are essentially monozygotic twins that are joined ‘in-utero’. This rare and heartbreaking phenomenon is elucidated by unfinished division of the fertilized ovum. The two individuals will be stuck to one another till death brings them apart. Unless they consider surgery, which carries cautioned risks. Connected at certain parts of the body, Conjoined twins usually share the same organs, limbs and tissues.
Polar Body Twins
Sometimes, the egg first splits. Later, each half collides with a sperm. These twins might be extremely alike in several ways including looks but their DNAs won’t be 100% compatible.
Mirror Image Twins
Coming from a single fertilized egg, mirror image twins develop reverse asymmetric features. This can be attributed by a delayed split of the fertilized egg, often surpassing a week. 25% of identical twins tend to be mirror image twins. One will be left-handed and the other, right handed. They may possess matching birthmarks on opposite sides of their bodies and suck opposite thumbs. The name springs from the fact that if face to face, they would be seeing exact mirror reflections of themselves!
Same Face, Different Birthday
What if a Labour begins at the peak hours of New Year’s Eve and the second twin is still on the way while the first one is already inhaling monoxides? The clock strikes twelve and Oopsie! Forget their birthdays being one day apart. Picture this- Molly was born in 1983 and her twin sister Polly in 1984! In certain cases, preterm labor forces the delivery of one baby while the other one’s can be controlled.
Identified in a pair of three-year old kids, these rare twins developed when two sperms fertilized one egg and later split. One twin was born a hermaphrodite while the other is anatomically male.
Very rarely but possibly a woman’s eggs can be fertilized by sperm from differing events of coitus. Suppose a woman engages in sexual intercourse with different partners, the twins could have separate biological fathers! This anomaly is defined as heteropaternal superfecundation.
When conjoined twins develop asymmetrically and a tinier and less formed twin depends on the stronger, larger twin to survive, they are known as parasitic twins. A variation of this called ‘fetus in fetu’ occurs when an abnormally formed mass of cells swell inside the body of its monozygotic twin. At times, the other twin doesn’t get sufficient nutrition from the placenta.
Twins of Different Races
Heteropaternal superfecundation speaks of cases wherein fraternal (dizygotic) twins exhibit differing racial characteristics. In one case, the differentiation was propelled by a lab mix-up during an in-vitro procedure. Also, in UK, two bi-racial parents conceived fraternal twin girl twins; one was dark coloured and the other white. This is a "one in a million" occurrence, where twins acquire different genetic characteristics from their mixed race.