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Introduction

The differences between identical and fraternal twins

Explanation of cellular division in twins

Frequently Asked Questions about twins

The implications of a multiple pregnancy

Possible complications in a multiple pregnancy

The following pictures and text explain the different ways that cellular division can occur in twins.

What's happening in each scenario? 

The Monozygotic twins are derived from only one egg and only one fertilization.

Scenario 1: If the separation takes place just after the first cellular division, then in this case both of the twins will have their own placenta and an amniotic sac each.

Scenario 2: The separation can also take place a little later in the development of the embryonic cells but before the blastocyte has fully defined the roles of each type of cell. At that moment, twins will be in the same placenta, but they will have 2 amniotic sacs.

Scenario 3: In the last of the identical twin scenarios, if the separation takes place at the stage when the amniotic bag is already being formed, then the twins will be in the same placenta, and in the same amniotic sac.

Scenario 4: Dyzygotic twins (fraternal twins), as shown in the diagram, are descended from a double ovulation and a double fertilization. The 2 eggs are completely independent. This configuration represents two thirds of all twin pregnancies.

Scenario 1 and Scenario 4 clearly have the same configuration at implantation into the uterus wall (bi-chorial and bi-amniotic) yet they are very different in that when the twins derive from one egg (monozygotic) they will be identical and the twins coming from two separate eggs will be fraternal.

New genetic techniques now allow for a Twin Zygosity Test

see also...

Introduction

The differences between identical and fraternal twins

Explanation of cellular division in twins

Frequently Asked Questions about twins

The implications of a multiple pregnancy

Possible complications in a multiple pregnancy

Related Articles:

Weekly development - Week 1

Weekly development - Week 2

Weekly development - Week 3

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