Twinning and Fetal Membranes

by Peter Ward, PhD

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    00:01 In this talk, we discuss the process of twinning and how different types of twins have a different set of membranes surrounding them as they develop.

    00:10 Now, most twins, two-thirds or so, are dizygotic; meaning they develop from completely separate fertilized eggs and they are also known as fraternal twins.

    00:20 So these twins develop as two completely distinct individuals in the womb.

    00:26 So each will form a separate blastocyst, a separate bilaminar, trilaminar embryo with their own amniotic cavity and their own placenta.

    00:36 And this placenta will implant separately into the womb.

    00:40 These twins are known as dichorionic and diamniotic signifying that they have two separate chorionic cavities, two separate amniotic cavities.

    00:50 And their placentae tend to be separate although occasionally, if they implant close to one another, they can share a bit of the placenta just due to the proximity of the two to one another.

    01:00 When it comes to identical twins we have a situation where a single zygote splits and instead of forming one individual, it?s going to form two or more individuals in the case of multiple births.

    01:14 There are three stages when this zygote can spilt and we?re gonna see how that impacts the membranes that surround each of these sets of twins.

    01:24 The earliest time that we can have separation of a zygote to form twins is at the two cell stage.

    01:31 So as soon as there are two cells we have the potential to have two separate individuals.

    01:34 Now, as these individuals develop they will form two separate blastocysts and two separate embryos with their own amnionic cavity.

    01:45 And essentially, they will have the same surrounding membranes as fraternal twins.

    01:51 They will have this completely separate amnion, chorion, and placenta.

    01:56 So they are also dichorionic and diamniotic.

    01:59 The next and most common time twins can form is during the blastocyst stage and at this point we move from the two cell stage to the eight cell stage and beyond, but at the blastocyst stage, instead of a single blastocyst embryoblast we have two embryoblast forming and because of that we?re gonna have two individuals develop.

    02:24 But, by the time we?re at the blastocyst stage, we have a surrounding chorionic membrane so there's only one chorion for these two people.

    02:33 They then develop to the embryonic stage and develop their own separate amniotic cavities.

    02:39 So these twins will be known as monochorionic and diamniotic.

    02:45 Meaning there is a thin chorionic membrane separating two amniotic cavities each one containing a separate individual.

    02:53 Last and least common will be separation of the developing zygote not at the blastocyst stage but at the bilaminar embryo stage.

    03:05 So we proceed with the person forming as a single individual all the way through the blastocyst stage, but here, when we form the bilaminar embryo a split in the primitive streak occurs that?s going to cause the formation of two separate individuals.

    03:21 Because of this, they already have a single chorionic cavities surrounding them.

    03:26 They have a single amniotic cavity developing above their shared epiblast, therefore, they?re going to be monochorionic and monoamniotic, and share a common amniotic chamber.

    03:40 These two will developed in close conjunction with each other sharing not only a placenta but the chorionic sac and the amniotic sac.

    03:49 One problem that can occur when a twinning event occurs at the bilaminar embryo stage, is that instead of a primitive streak splitting to form two separate individuals, the primitive streak can split but remain attached in one region or another.

    04:04 When this happens, we can have conjoined twins develop, meaning that they?re fused at some portion of their body, and this fusing can occur almost anywhere in the body; be at the pelvis, the abdomen, the thorax, the head, or even two separate heads growing off a common body.

    04:22 This is due to a twinning event that incompletely separates at the primitive streak.

    04:26 Other problems that can occur in twinning are known as types of parasitic twins.

    04:32 In this case, twins developed but one member, more or less, wraps around and engulfs the other member and thereafter, one twin will develop off of the larger twin.

    04:45 In this case we may have a parasitic twin forming with parts of the body extending from its other twin and not developing completely.

    04:54 In some instances, a parasitic twin can form entirely inside its other sibling, and in this case we have a condition called fetus-in-fetu where the larger twin completely enveloped the other and the fetus that is developing inside the other maybe completely reliant upon the larger twin for blood, nutrition, and etc.

    05:18 Now, it?s not uncommon for twins to be slightly undersized compared to children who are carried to term singly and is typically not a problem.

    05:26 However, problems can occur when twins have unequal blood supply.

    05:32 This is known as twin transfusion syndrome, and this occurs when one twin is receiving more blood from the placenta than its sibling.

    05:40 The twin with too little blood will often develop anemia and grow more slowly than the larger twin.

    05:46 The larger twin however, maybe receiving so much blood that it actually makes it hard for the heart to pump against that increased volume and lead to congestive heart failure.

    05:55 So if discovered early, it may need to be surgically corrected so that equal blood flow is established between the two siblings.

    06:03 Thank you very much.

    About the Lecture

    The lecture Twinning and Fetal Membranes by Peter Ward, PhD is from the course Conception, Implantation and Fetal Development.

    Included Quiz Questions

    1. Monochorionic and diamniotic, forming at the blastocyst stage
    2. Dichorionic and diamniotic, forming at the two-cell stage
    3. Dichorionic and diamniotic, forming at the blastocyst stage
    4. Monochorionic and monoamniotic, forming at the bilaminar embryo stage
    5. Monochorionic and monoamniotic, forming at the blastocyst stage
    1. A single primitive streak forms and develops a split somewhere along its length.
    2. No primitve streak forms.
    3. Two primitive streaks form and then combine to form a single streak.
    4. Two primitive streaks form and full separate from each other.
    5. Aneuploidy
    1. Twin transfusion syndrome
    2. Conjoined twins
    3. Parasitic twins
    4. Fetus-in-fetu
    5. Bilaminar malformation

    Author of lecture Twinning and Fetal Membranes

     Peter Ward, PhD

    Peter Ward, PhD

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