Twinning

Introduction

Singleton pregnancy is typically observed because, like humans, ovules are only released once throughout a menstrual cycle. Multiple pregnancies refer to when a mother gives birth to two or more children during a single pregnancy. It's typical for dogs, cats, and many other mammals to become pregnant multiple times. Twining is the term used when a mother has two children during a single pregnancy. The twin incidence in IVF is 21 twins per 1000 live births, increasing the likelihood of multiple pregnancies.

Classification of twins

According to the number of children born, multiple pregnancies are categorized as follows:

• 
  Twins: two children
• Triplet: three children
• Quadruplets: four children
• Quintuplets: five children

The following categories apply to multiple pregnancies depending on the genetic relationship of the children:

• Monozygotic twins: These twins are created when one sperm fertilizes a single ovum (one zygote).
• Dizygotic twins: These twins are created when two different oocytes are fertilized by two distinct sperm (two zygotes).
• Trizygotic twins: These are created when numerous ova are fertilized by various sperm (3 zygotes).

Dizygotic (Fraternal) Twins:

Dizygotic twins are created when two separate eggs are fertilized by two different sperms (two zygotes). In contrast to having an incidence ratio of 7-10 per 1000 births, the incidence increases with age.

Features

• The phenotypical differences between the dizygotic twins may potentially include a different gender (one male and the other female offspring).
• The dizygotic twins are genetically distinct and do not share a placenta, chorion, or amnion (unless sporadically).
• Each of them has a unique gestational sac.
• Because placentae can sometimes be close to one another, merged placentae may occasionally be seen.
• Human twins are dizygotic in about 2/3 of cases.

Monozygotic (Identical) Twins:

Monozygotic twins are formed when one oocyte is fertilized by one sperm (single zygote). As a result, a blastocyst with an inner cell mass is formed. Monozygotic twins are created when this core clump of cells, called an embryoblast, divides into two.  The incidence rate is three per 1,000 live births.

Features:

• Monozygotic twins exhibit similar phenotypes (they have the same gender).
• Their genetic makeup is comparable and About one-third of human twins are monozygotic twins, which means they do not share a chromosome or an amniotic sac or placenta.

Classification of twins: According to degree of separation 

Monozygotic twins can be classified on the degree of separation as follows:

• Dichorionic Diamniotic Twins: Also known as bichorial, diamniotic twins, these are birth defects. After the first few divisions, the cells of the zygote separate to form two embryos with distinct chorionic and amniotic sacs in monozygotic twins. Dichorionic and diamniotic twins are produced when cells separate on the third day of fertilization. These twins' placentas are separate, which occurs in 25% of twin pregnancies.
• Monochorionic Diamniotic Twins: They are also known as monochorial, diamniotic twins, and their mechanism of development is the separation of the blastocyst's inner cell mass between the fourth and seventh day. These twins have a single placenta and a shared chorionic sac, but two amniotic sacs. Twin pregnancies have an incidence ratio of 70–75%.
• Monochorionic Monoamniotic Twins: Also known as monochorial twins, monoamniotic twins are born with a shared placenta, chorionic cavity, and amniotic cavity when the inner cell mass separates on the ninth day of fertilization. Pregnancies with monozygotic twins have an incidence ratio of 1% to 2%.
• Conjoined (Siamese) Twins: Monozygotic twins can become conjoined twins with joined bodies if they are separated after the 12th day of fertilization.

Conjoined twin's classification on the basis of location and degree of fusion:

• 
  Craniopagus: is a head fusion (skulls).
• Thoracopagus: Up to the umbilicus, the anterior thoracic and abdominal walls fuse.
• Omphalopagus: The joining of the lower abdominal walls
• Cephalothoracopagus: The fusion of the thoracic and head walls.
• Pyopagus: Fusion of the sacral areas
• Ischiopagus: Fusion of the pelvis. Traditionally, the spinal axis of these twins is united at 180 degrees.
• Parasitic twin: a conjoined twin with a larger and a smaller (parasitic) twin.

References:

• Sontakke Y. Textbook of human embryology: With clinical cases and 3D illustrations. New Delhi: CBS Publishers & Distributors Pvt Ltd; 2019.
• Sadler TW. Langman's medical embryology: T.W. Sadler.. 13th ed. Philadelphia: Lippincott Williams & Wilkins; 2015.