Zygosity refers to whether twins share the same genes or not. Identical twins develop from a single fertilized egg that splits into two embryos. They have the same DNA and are always the same sex. Fraternal twins develop from two separately fertilized eggs and share on average 50% of their DNA, just like non-twin siblings. Fraternal twins can be the same or different sexes. Knowing if twins are identical or fraternal is important for understanding twin similarities and differences.
How is twin zygosity determined?
There are several ways to determine if twins are identical or fraternal:
Physical characteristics
Identical twins tend to share physical characteristics like hair color, eye color, height, and build more than fraternal twins. However, fraternal twins can also look very similar. Comparing physical traits alone is not a very reliable method.
Blood type
If twins have different blood types, such as one being type A and the other type B, they must be fraternal. But if they share the same blood type, they could be either identical or fraternal. Blood type alone cannot distinguish identical from fraternal twins.
Sex
Identical twins are always the same sex, while fraternal twins can be the same or different sexes. About 1/3 of fraternal twins are opposite sex. However, the same sex does not definitively mean identical twins.
DNA analysis
Analyzing the twins’ DNA provides a conclusive determination of zygosity. Identical twins share 100% of their DNA. Fraternal twins share about 50% on average. DNA can be obtained from a cheek swab and analyzed to compare genetic markers.
What are some statistics on twin zygosity?
Some key statistics on the zygosity of twins:
- About 1/3 of twins are identical.
- About 2/3 of twins are fraternal.
- Of male/female twins, about half are identical and half fraternal.
- Of male/male twins, about 1/3 are identical and 2/3 fraternal.
- Of female/female twins, about 2/3 are identical and 1/3 fraternal.
This table summarizes the statistics:
| Twin Type | Percent Identical | Percent Fraternal |
|---|---|---|
| Male/Female | 50% | 50% |
| Male/Male | 33% | 67% |
| Female/Female | 67% | 33% |
As shown, the ratio of identical to fraternal depends on the sex of the twins, but overall about 1/3 of twins are identical.
What causes twins?
Twins occur due to two main factors: hyperovulation and splitting of the embryo.
Hyperovulation
Hyperovulation is when a woman releases multiple eggs during ovulation instead of just one. This can lead to fraternal twins if more than one egg is fertilized. Factors that increase hyperovulation include:
- Family history – Having a mother or sister who conceived twins hyperovulation and twinning tend to run in families.
- Fertility treatments – Medications used for ovarian stimulation can cause the ovaries to release multiple eggs.
- Age – Women over 35 are more likely to hyperovulate.
- Race – African ancestry increases the chance of hyperovulation.
Since fraternal twins come from separate eggs, hyperovulation is the cause of fraternal twinning.
Splitting of the embryo
Identical twins occur when a single fertilized egg splits into two separate embryos very early in development, about 3-4 days after conception. The two embryos have the same DNA and develop into identical twins. It is not well understood why embryo splitting occurs, but it is a random event and not genetic. After splitting, the two embryos implant separately into the uterus and develop into twins who share 100% of their genes.
What factors make someone more likely to have twins?
While any woman can spontaneously have identical twins, certain factors make having fraternal twins more likely:
- Family history – Having a close relative like a sister or mother who gave birth to fraternal twins raises your odds.
- Age – Women over 30 and especially over 35 have higher rates of hyperovulation.
- Race – Women of African descent naturally have higher rates of twinning.
- Body size – Tall or overweight women produce more hormones that can spur multiple eggs.
- Prior births – Having given birth before ups the chances of twins.
- Fertility assistance – Drugs and procedures used in infertility treatment often cause hyperovulation.
So women with a family history of twins, over age 30, of African ancestry, plus sized, who have given birth before, and use fertility treatments are all more likely to conceive fraternal twins by releasing multiple eggs. However, identical twins are still random chance.
Do identical twins have the same placenta and amniotic sac?
Placenta
With identical twins, about 66% share the same placenta. The placenta delivers oxygen and nutrients from mom to baby. Sharing a placenta is called being monochorionic.
About 33% of identical twins have separate placentas. This is called being dichorionic. Having individual placentas reduces competition for resources.
Fraternal twins always develop separate placentas since they come from different eggs.
Amniotic sac
About 75% of identical twins share the same amniotic sac or are monoamniotic. They float together in the same sac of amniotic fluid.
About 25% have separate sacs or are diamniotic. Each fetus has its own amniotic sac and space, although they share the placenta.
Fraternal twins always have separate amniotic sacs.
So most identical twins share both the placenta and amniotic sac, while fraternal twins never share either. But not all identical twins share both structures.
What are some unique health risks for identical vs. fraternal twins?
Identical twins
Being monochorionic and monoamniotic puts identical twins at higher risk for:
- Twin-to-twin transfusion syndrome – An imbalance of blood flow through the shared placenta.
- Cord entanglement – The umbilical cords can wrap around each other.
- Premature birth – Crowding in the womb increases odds of early labor.
Identical twins are also more likely to suffer congenital disabilities if there is a genetic mutation early in development that impacts both fetuses.
Fraternal twins
Fraternal twins do not share placentas or amniotic sacs, so they avoid risks like twin-to-twin transfusion. However, because fraternal twins result from hyperovulation, the mother faces higher chances of:
- Pregnancy complications like preeclampsia and gestational diabetes
- Premature labor and delivery
- Need for cesarean section birth
Carrying multiples also leads to greater fatigue and likelihood of bed rest for the mother. So both identical and fraternal twins pose unique risks during pregnancy. Proper prenatal monitoring is essential.
How similar are identical vs. fraternal twins?
Appearance
Identical twins typically appear nearly indistinguishable. They have the same facial features, eye and hair color, height, and build. Minor differences in freckles or scars can emerge over time.
Fraternal twins appear as alike as any other sibling pair. They may share eye color or general look, but small differences will be noticeable. Fraternal twins of different sexes will clearly look distinct as they age.
Interests and abilities
Identical twins reared together show staggering similarities across a wide array of traits like personality, intelligence, mental health, talents, and more. Their shared genes heavily shape these traits.
Fraternal twins are similar to the extent that all siblings are. They share personality traits, interests, and abilities at the same level as any brother or sister pair. Their genes only partially overlap.
Health
Identical twins share all disease-related genes. So if one twin develops an inherited health condition, the other is at very high risk as well. This includes conditions like heart disease, cancer, diabetes, and mental illness.
Fraternal twins again share genetic risks at the same level as regular siblings, about 50% genetic overlap. If one develops an inherited condition, the other’s risk is elevated but not nearly as high as for identical twins.
So identical twins appear astonishingly alike and share nearly all traits. Fraternal twins only resemble each other as much as any brother and sister.
How is twin zygosity tested prenatally?
Today zygosity can be determined before birth by:
Ultrasound
Sonograms as early as 8 weeks gestation can reveal if twins have separate placentas and amniotic sacs. This indicates they are fraternal. Finding a single placenta and sac suggests identical twins. However, ultrasound alone is not completely definitive.
Chorionicity blood testing
Blood tests can detect specific hormone levels that reveal if twins share a placenta or not. This helps distinguish identical from fraternal twins. A blood draw from mom starting at 8-10 weeks can test chorionicity.
Amniocentesis
This test draws amniotic fluid from each sac which contains fetal cells. DNA analysis of the cells can conclusively determine zygosity. However, amniocentesis does carry a small risk of miscarriage. It is not routinely done just to determine zygosity.
Non-invasive prenatal testing (NIPT)
New cell-free DNA tests allow zygosity testing as early as 9-10 weeks with no miscarriage risk. DNA shed from the fetuses into mom’s blood is analyzed to assess genetic similarities. NIPT is rapidly becoming the standard for prenatal zygosity detection.
So today zygosity can be accurately determined prenatally without any harm to the twins using cell-free DNA tests.
What is a twin pregnancy like for mom?
Moms pregnant with twins face special challenges:
Greater fatigue
Carrying two babies puts greater metabolic demands on mom’s body. Feeling very tired as well as muscle aches and back pain are common. Rest is important.
Higher caloric needs
A twin pregnancy requires at least 500 extra calories per day, and more if mom was underweight to start. Eating frequent, nutritious snacks and meals is vital.
More weight gain
Healthy weight gain for a twin pregnancy is 35-45 pounds compared to 25-35 pounds with a singleton. This helps nurture the babies’ growth and development.
Possibility of bed rest
If complications like preterm labor arise, the doctor may recommend full or partial bed rest to reduce risks. This may involve stopping work and limiting activity.
Higher chance of interventions
Twin deliveries have elevated risks of needing interventions like emergency c-sections, induction, or vacuum/forceps assistance to deliver safely. Moms should discuss these possibilities with their doctor.
So a twin pregnancy demands more from mom’s body. Following doctor’s orders and getting support can result in the best outcome.
Do twins have their own unique identities?
Twins often share deep bonds, but each one develops a distinct identity:
Different looks
Even identical twins have subtle physical differences like distinct facial shapes, hair whorls, freckles, scars or birthmarks that set them apart in appearance.
Different personalities
Though shaped by the same genes, experiences shape personality. Twins develop their own unique quirks, temperaments, strengths and weaknesses over time.
Different interests
Twins may share certain interests but often diverge and develop their own hobbies, favorite subjects, and passions as they follow their inborn talents.
Different friend groups
As twins age, they create their own social circles at school or in extracurriculars based on mutual bonds. These distinct friendships help shape identity.
Different accomplishments
Twins blaze their own trails and have different academic performances, career pursuits, relationship experiences, and life milestones that add to their identity.
So while deeply connected, twins are distinct individuals. Zygosity does not dictate their identity. Nurturing each twin’s uniqueness is key for parents.
Conclusion
Understanding zygosity provides insight into twins’ shared traits as well as uniqueness. Determining zygosity is important for predicting health risks and guiding parenting approaches. While identical twins share remarkable similarities, all twins forge their own paths in life. With the right support, twins of any zygosity can achieve their full potential and develop independent identities. The twin bond remains powerful, but should not overshadow celebrating each one as a wonderfully distinct individual.