Semi-Identical Twins

Medically Reviewed by Dr. Silvana Chedid on August 31, 2026

Semi-identical twins, also called sesquizygotic twins, share all of their mother’s DNA but only part of their father’s DNA, which places them genetically somewhere between identical and fraternal twins. Only two such cases have ever been confirmed anywhere in the world. The second was identified in Brisbane, Australia, and reported in the New England Journal of Medicine in 2019, marking the first time this type of twinning had been detected during pregnancy rather than discovered afterward through genetic testing.

Patients who come to us for a Miami consultation ask a wide range of questions about twins, from the odds of conceiving multiples naturally to how fertility treatment changes those odds. Dr. Silvana Chedid has spent more than 30 years studying these questions in her reproductive medicine practice, and we think understanding how twins actually form, and what does and does not raise your chances of carrying more than one baby, helps you set realistic expectations before starting any treatment plan.

What Makes Semi-Identical Twins Genetically Unique

In the confirmed Australian case, researchers found that the twins carried 100 percent of their mother’s DNA in common but only about 78 percent of their paternal DNA, an overlap that sits between the near-total similarity of identical twins and the roughly 50 percent overlap typical of fraternal twins. Doctors believe this happens when a single egg is fertilized by two sperm at nearly the same moment, before the fertilized cell divides.

That kind of fertilization normally produces three sets of chromosomes instead of two, a combination researchers have described as typically incompatible with life, since embryos formed this way usually do not survive. In the rare instance where the embryo does survive and splits into two, the resulting twins end up with paternal chromosomes distributed unevenly between them, which produces the partial genetic overlap seen in sesquizygotic twins.

How Ordinary Identical and Fraternal Twins Form

Outside of these rare sesquizygotic cases, twins form in one of two typical ways. Identical, or monozygotic, twins begin as a single egg fertilized by a single sperm. The fertilized egg then splits into two separate embryos that carry virtually the same genetic material and are always the same sex.

Fraternal, or dizygotic, twins begin very differently. Two separate eggs are released during ovulation, and each is fertilized by a different sperm, producing two siblings who share roughly half their genes, the same as any brother or sister, and who can be the same sex or different sexes. Fraternal twins account for the large majority of twin pregnancies, and their likelihood is shaped by family history, maternal age, and fertility treatment, while identical twinning happens more or less at random and is not linked to heredity.

What Raises Your Chances of Carrying Twins

Maternal age is one of the clearest factors. Twin birth rates have historically been higher among women in their late 30s and 40s, largely because hormonal changes associated with age make it more likely that more than one egg is released during a single cycle. Family history plays a role as well, since a tendency toward releasing multiple eggs can run in families.

Fertility treatment is another significant factor, and it works differently than the other two. Rather than changing how many eggs your body releases on its own, treatments like in vitro fertilization and ovulation induction can directly affect how many embryos are available or transferred in a given cycle. Ovulation-stimulating medications can prompt more than one egg to mature in a single cycle, and if more than one is fertilized, a fraternal twin pregnancy can result even without any embryo transfer being involved.

None of these factors guarantee or rule out a twin pregnancy on their own. Maternal age, family history, and fertility treatment each shift the odds somewhat, but the specific likelihood in any individual case depends on a combination of health history, hormone levels, and the type of treatment being used, which is why we walk through this in detail during a consultation rather than quoting a single generic number.

How Fertility Treatment Relates to the Odds of a Multiple Pregnancy

CDC data published in 2026 shows twin birth rates actually declining over the past decade, falling from 48.6 to 35.4 per 1,000 births among women ages 35 to 39, and from 66.0 to 35.6 per 1,000 births among women 40 and older, even as more women pursue fertility treatment later in life. Fertility researchers widely attribute this decline to a shift in how IVF is practiced.

In the early years of IVF, physicians often transferred multiple embryos in a single cycle because many did not survive, which meant twins and triplets were common outcomes. Improvements in embryo culturing, freezing, and genetic testing have made it possible to identify the strongest single embryo for transfer, and professional guidelines now recommend that most patients with a favorable prognosis receive only one embryo per cycle. Even with single embryo transfer, IVF patients still carry a small chance, generally cited around 1 to 3 percent, of the transferred embryo splitting into identical twins on its own.

How We Approach Embryo Transfer Decisions at Chedid Grieco

Because a multiple pregnancy carries higher risks for both parent and babies, deciding how many embryos to transfer is one of the more important choices in any IVF cycle. Our team builds each transfer plan around your individual health history, embryo quality, and family-building goals rather than a single standard protocol, which is part of why we describe our approach as boutique and customized rather than one-size-fits-all.

For many patients, that means recommending a single embryo transfer to help manage the odds of a multiple pregnancy, paired with genetic screening to help select the embryo with the strongest chance of a healthy outcome. That said, single embryo transfer is one option among several, not a fixed rule, and some patients, particularly those with specific medical or family-building circumstances, may discuss a different approach with our team during their consultation.

Because consultations happen in Miami while treatment is carried out at our FDA-licensed, Brazil-regulated clinic, we can also walk you through what that process looks like logistically before you commit to a plan, an approach we cover in more detail on our fertility tourism page. Reviewing your options in person, before any embryos are created or transferred, gives you time to ask questions and make a decision that fits your own health history and goals rather than a one-size-fits-all recommendation.

Frequently Asked Questions About Twins and Fertility Treatment

Dr. Silvana Chedid has practiced reproductive medicine for more than 30 years and was the physician who introduced intracytoplasmic sperm injection, ICSI, to Brazil. She has helped build nearly 9,000 families throughout her career, leading a female-led, LGBTQI+-supportive practice that completes roughly 1,200 treatment cycles a year at our FDA-licensed and NYDH-licensed, Brazil-regulated clinic.

If you are exploring fertility treatment and want to understand your own likelihood of a multiple pregnancy, or simply want a clear, personalized explanation of your options, we encourage you to schedule a Miami consultation with our team. Reach out through our Contact form to get started.