Roberta Spiro

Can twins read each other’s minds?

Twins share a special connection beyond that of ordinary siblings. Some people believe that this unique twin bond is endowed with extraordinary supernatural qualities. There are many stories that support this idea. For example, twins will report that one twin experienced a physical sensation of something that was happening to their twin, such as a […]

What are mirror image twins?

Mirror image twins are a type of identical (monozygotic or MZ) twins. The term “mirror image” is used because the twins, when facing each other, appear as matching reflections. They have the same physical features but some are opposite. For example, if one twin is right-handed the other twin may be left-handed. Their hair whorls […]

Can twins have different birthdays?

Yes! Twins are defined as children produced in the same pregnancy. Usually they are delivered only a few minutes or hours apart. But they can have different birthdays. This most commonly happens when labor and delivery begins before midnight on one day and ends after the clock changes to the next day. If that day […]

Twins run in my family. Do I have an increased chance of having twins?

There are several factors to consider in answering this question. Are the twins in the family a result of fertility treatments? These treatments include fertility drugs and assisted reproductive technologies such as in vitro fertilization. The use of such fertility treatments has caused a skyrocketing in the number of fraternal (dizygotic or DZ) twin births […]

Can identical twins look different?

Yes! Identical twins came from the same sperm and egg, so they have the same chromosomes and genes. But there are environmental differences that can affect the way they look and behave. For example, one twin may have been positioned in the womb in a way that she got more nutrients or blood supply and may […]

What factors are related to fraternal twinning?

The birth rate of identical (monozygotic or MZ) twins is the same all over the world. It is approximately four per thousand births. But the birth rate of fraternal (dizygotic or DZ) twins varies. DZ twins occur when two eggs are released in the same menstrual cycle and are fertilized by two different sperm. The […]

Why is the number of twins increasing?

The number of twin births has increased dramatically over the past 30 years. A recent government study showed that the twin birth rate rose 76% from 1980-2009! In 1980, one in every 53 babies born in the US was a twin. In 2009, one in every 30 babies was a twin! This increase is due to […]

What is twin-to-twin transfusion syndrome?

Twin-to-twin transfusion syndrome (TTTS) is a rare condition that occurs only in identical twins when they are in the womb. The cause is unknown. It occurs when blood moves from one twin to the other. The twin that loses the blood is called the donor twin, and the twin that receives the blood is called […]

Do female identical twins appear more different than male identical twins?

Yes, female identical (MZ) twins can appear more different than male identical twins! The epigenetic factors that can cause identical twins of either sex to look different are discussed in the answer to the question “Can identical twins look different?” In addition, the sex chromosomes are responsible for the greater difference in appearance of some […]

What is a hidden twin?

In the past, before ultrasound examinations were a common part of prenatal care, parents were sometimes surprised when two babies were born even though only one fetal heartbeat was heard during the pregnancy. Today, a routine prenatal ultrasound examination can identify a twin pregnancy. If two fetuses are seen by ultrasound, the woman is expecting […]

Can twins have different fathers?

Yes! Fraternal (DZ) twins develop when a woman releases two eggs instead of one during her monthly menstrual cycle. When this occurs and each egg is fertilized by sperm cells from the same man, DZ twins with the same father are created. While rare, it is possible that each egg could be fertilized by a […]

Can twins be of different races?

Racial differences involve many genes. The genetics of racial appearance is very complex. Below is a simplified answer to this question. Fraternal (dizygotic or DZ) twins that appear to be of different races have been described. This could happen in several different ways: Twins that appear to be of different races could result from heteropaternal […]

Do identical twins have identical fingerprints?

No! Studies have concluded that, even though the fingerprints of identical (MZ) twins may be very similar, they are not identical. MZ twins have a very high correlation of loops, whorls and ridges. But the details (for example, where skin ridges meet, divide into branches, or end) differ between MZ twins. MZ  twins share the […]

Are there more male twins or female twins?

Among non-twin births, males are slightly (about five percent) more common than females. In the US, 105 non-twin males are born for each 100 non-twin females. However, males are slightly more likely than females to die in the womb. And because the death rate in the womb is higher for twins than for singleton births, […]

Can fraternal twins look so similar that they are mistaken for identical?

Yes, it is possible for same-sex fraternal twins to look extremely similar. Like any siblings, fraternal twins are the products of two separately fertilized eggs from the same mother and father. They are genetically as similar as other non-twin siblings. By chance, they could look very similar, or very different, just as non-twin siblings can […]

Does Identical Twinning Run in Families?

Fraternal (dizygotic or DZ) twins sometimes run in families. In the past, researchers believed that identical (monozygotic or MZ) twins do not run in families. They said that MZ twinning was a random event that occurs equally as often in all places and populations around the world. But many people know or have heard of […]

How often do twin births occur?

In the US, there were 135,336 births in twin deliveries in 2014, the most recent year for which government statistics are available. This represents an increase from 2013, but is lower than the peak years of 2006-09. In Washington State, there were 2,660 births in twin deliveries in 2014. The birth rate for identical (monozygotic, or […]

I am a twin. Do I have an increased chance of having twins?

It was once believed that identical (monozygotic or MZ) twins occured at random. There is now some evidence to suggest that MZ twins may run in families, but this is very rare. In general, it is fraternal (dizygotic or DZ) twins that run in families. It is believed that there are genetic factors that can […]

Can a male/female twin pair be identical?

The term “identical twins” is generally used as a synonym for “monozygotic (MZ) twins.” Both terms assume that that the twins developed from the same fertilized egg (zygote), have identical DNA, and therefore look identical. But, as explained here, identical twins do not always look exactly the same. And male/female twins certainly don’t look identical! […]