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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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, […]
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 […]
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 […]
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 […]
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 […]
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! […]
No! In about 21% of identical (monozygotic or MZ) twin pairs, one twin is right-handed and the other is either left-handed or ambidextrous. Since identical twins share identical genes, this is evidence that handedness is not a totally genetic trait. Left-handedness is more likely to occur in twins than in single individuals. Only about 10% […]
If your twin develops certain diseases, you may have an increased risk of developing that disease. This risk may be higher if you are identical twins than if you are fraternal twins. If you would like additional information about how your twin’s health might impact your own health, contact your health care provider or a […]
