Illustration of a blastocyst dividing, representing monozygotic twinning in IVF.

The IVF Twinning Mystery: Why Blastocyst Transfers Can Result in Multi-Chorionic Pregnancies

Elliot Brynn in Health & Wellness August 2025 • 5 min read.

"Challenging the Conventional Wisdom: A Deep Dive into Monozygotic Multi-Chorionic Pregnancies Following Single Embryo Transfer at the Blastocyst Stage"

For decades, the understanding of how twins form after fertilization has been relatively straightforward: the timing of when a single embryo splits determines the type of twin gestation. Dichorionic-diamniotic (DC-DA) twins, where each fetus has its own placenta and amniotic sac, were thought to arise only from embryos splitting within the first three days after fertilization. After that, the thinking went, only monochorionic twins (sharing a placenta) could develop.

However, in vitro fertilization (IVF) has thrown a wrench into this long-held belief. While IVF is known to increase the overall rate of multiple pregnancies (mostly dizygotic, or fraternal twins, due to transferring multiple embryos), it's also been linked to a higher incidence of monozygotic (identical) twins. Even more intriguing is the emerging evidence that blastocyst transfer, a common IVF practice, can sometimes lead to multi-chorionic pregnancies challenging the existing dogma.

This article delves into the fascinating phenomenon of multi-chorionic pregnancies arising from single blastocyst transfers. It examines a case series and reviews existing literature to shed light on this unexpected outcome, exploring the potential factors involved and what it means for our understanding of twinning and IVF.

Unraveling the Blastocyst Twinning Enigma: Challenging the Day 3 Split Theory

Illustration of a blastocyst dividing, representing monozygotic twinning in IVF.

The conventional wisdom, initially presented by Corner in 1955, dictates that DC-DA twins can only form if the embryo divides within the first three days post-fertilization. Monochorionic-diamniotic (MC-DA) twins were believed to result from division between days 4 and 8, and monochorionic-monoamniotic (MC-MA) twins from division between days 9 and 12. Conjoined twins were considered possible only after day 13.

But what happens when a single blastocyst, typically transferred around day 5, leads to a multi-chorionic pregnancy? This is where the established theory faces a challenge. Several factors may be at play, including:

Micromanipulation Techniques: Procedures like intracytoplasmic sperm injection (ICSI), assisted hatching, and preimplantation genetic screening (PGS) involve manipulating the zona pellucida (the outer shell of the embryo), which some studies suggest could increase the risk of monozygotic twinning. The mechanism is thought to be either a weakening of the zona, leading to easier splitting, or changes in cell-to-cell adhesion within the embryo.
Extended Culture: Growing embryos in the lab for longer periods (to the blastocyst stage) might also contribute. Some propose that the culture media can harden the zona pellucida, causing the inner cell mass to pinch off and split during hatching.
Atypical Hatching: In rare cases, the blastocyst might not hatch normally through a large slit in the zona. Instead, it could herniate through a small hole, leading to division into two halves, each with its own trophoectoderm (outer layer) and inner cell mass.

The fact that embryologists have not observed embryos spontaneously splitting before the blastocyst stage in IVF labs further underscores the uniqueness of this phenomenon. While artificial separation of blastomeres at the 4-cell stage can lead to separate blastocysts, spontaneous early splitting is rarely, if ever, seen.

Re-evaluating Twinning Theories in the Age of IVF

The cases of multi-chorionic pregnancies arising from single blastocyst transfers challenge the long-held Corner's theory of embryonic division. While micromanipulation of the zona pellucida, extended culture, and rare occurrences of atypical hatching may all play a role, there is still much to learn about the precise mechanisms at play.

Continued scientific discussion and investigation are crucial to refine our understanding of monozygotic DC-DA twinning, particularly in the context of IVF. Advanced research may reduce the increased risk of monozygotic multiplets linked with IVF technologies. By carefully analyzing and considering these new realities, medical professionals can improve IVF outcomes, potentially reducing the occurrence of monozygotic multiples.

It's time to re-evaluate the traditional framework of twinning and accept that our understanding is evolving, due in large part to the advanced techniques used in modern fertility treatments.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1007/s10815-018-1329-8, Alternate LINK

Title: Multi-Chorionic Pregnancies Following Single Embryo Transfer At The Blastocyst Stage: A Case Series And Review Of The Literature

Subject: Genetics (clinical)

Journal: Journal of Assisted Reproduction and Genetics

Publisher: Springer Science and Business Media LLC

Authors: Viji Sundaram, Salustiano Ribeiro, Martha Noel

Published: 2018-10-25

Everything You Need To Know

What is the core concept related to twinning that is being challenged?

The established understanding, known as Corner's theory, suggests that the timing of the embryo's split determines the type of twins. This theory posits that Dichorionic-diamniotic (DC-DA) twins result from splitting within the first three days after fertilization, Monochorionic twins form later. This theory is challenged by In Vitro Fertilization (IVF) practices, specifically blastocyst transfers, which sometimes lead to multi-chorionic pregnancies despite the embryo being transferred at a later stage.

What is a single embryo transfer at the blastocyst stage, and why is it important?

A single embryo transfer using the blastocyst stage is when an embryo is grown in a lab to a specific stage (blastocyst) and then transferred. This is done around day 5 of development. While generally considered a safe and effective method in IVF, it has been linked to an increased incidence of monozygotic (identical) twins. The significance lies in its challenge to the established Corner's theory, which predicted twinning types based on the timing of embryo division.

What are the potential factors that could explain multi-chorionic pregnancies after single blastocyst transfers?

Several factors are thought to play a role in the phenomenon of multi-chorionic pregnancies arising from single blastocyst transfers. These include micromanipulation techniques like intracytoplasmic sperm injection (ICSI), assisted hatching, and preimplantation genetic screening (PGS) that manipulate the zona pellucida, the outer shell of the embryo. Extended culture, where embryos are grown for longer periods, and atypical hatching processes are also considered potential factors. These factors challenge the existing dogma about when and how twins form.

How can micromanipulation techniques affect the chance of twins?

Micromanipulation techniques like ICSI, assisted hatching, and PGS can potentially increase the risk of monozygotic twinning. These procedures involve manipulating the zona pellucida, the outer shell of the embryo. This manipulation may weaken the zona, making it easier for the embryo to split, or it might alter cell-to-cell adhesion. The implications are significant because they suggest that certain IVF procedures could directly influence the likelihood of identical twins. These actions may indirectly affect established beliefs about twin development.

How can the extended culture of embryos in the lab affect the process of twin development?

Extended culture, where embryos are grown in the lab for a longer duration, might contribute to multi-chorionic pregnancies. Some researchers propose that the culture media could cause the zona pellucida to harden. This hardening could cause the inner cell mass to split during hatching. This challenges traditional understanding. If the zona pellucida's condition impacts twinning, it suggests that the lab environment and procedures have a more significant role in the process than previously thought. The implications could lead to adjustments in the lab practices to minimize this risk.