Multiples What Are Fraternal Twins? By Pamela Prindle Fierro Pamela Prindle Fierro Pamela Prindle Fierro is the author of several parenting books and the mother of twin girls. Learn about our editorial process Updated on November 10, 2020 Fact checked Verywell Family content is rigorously reviewed by a team of qualified and experienced fact checkers. Fact checkers review articles for factual accuracy, relevance, and timeliness. We rely on the most current and reputable sources, which are cited in the text and listed at the bottom of each article. Content is fact checked after it has been edited and before publication. Learn more. by Andrea Rice Fact checked by Andrea Rice Facebook LinkedIn Twitter Andrea Rice is an award-winning journalist and a freelance writer, editor, and fact-checker specializing in health and wellness. Learn about our editorial process Print Table of Contents View All Table of Contents What Are Fraternal Twins? How They Are Formed Types of Fraternal Twins Characteristics Health Risks Coping What Are Fraternal Twins? Fraternal twins, or dizygotic twins, happen when two different eggs are fertilized by two sperm. "Di" means two and zygotic refers to the zygote, the egg fertilized by the sperm that will develop into an embryo and grow into a baby. The majority of twins are fraternal twins (twins that are more like siblings born at the same time). Monozygotic twins, or identical twins, come from a single egg and sperm that splits into two after conception. Learn more about fraternal twinning, from how they form to how they relate to each other, as well as how they differ from identical twins. How Fraternal Twins Are Formed Fraternal twins form in the same way that all humans do, by the union of sperm and egg. Dizygotic twinning occurs when more than one egg is released during ovulation. This is known as hyperovulation. There are numerous reasons for hyperovulation, including: Height: Taller women have a higher-than-average rate of twin pregnancies.High body weight: Extra fat stores produce increased levels of estrogen.Maternal age: As you grow older, you're more likely to hyperovulate.Multiple births: Women who have previously given birth to several children may be more likely to have twins.Race: Highest rates of twinning are found in Central African populations, while Asia and Latin America have the lowest rates of twinning. There are weaker associations with using birth control pills, folic acid, and the season of the year. None of these factors have been proven to increase the chances of having identical twins. Genetics A tendency toward hyperovulation can be a genetic trait. In this way, fraternal twinning can be hereditary. A woman who has the gene for hyperovulation can pass it down to her daughter. Then, the daughter's chances of having twins are increased. Because men carry both X (female) and Y (male) chromosomes, they can also hold the trait for hyperovulation. They can pass it along to their daughters too, increasing their daughters' chances of having fraternal twins. If there are fraternal twins on both the mother's and father's sides, your odds for twins goes up even higher. However, having the gene for hyperovulation does not increase a man's chances of fathering fraternal twins. If a man carries the gene, it doesn't change the ovulation pattern of the mother of his children. She has her own genes governing her ovulation. Instead, it would be his daughter who inherits it through his genes. That is why twins are sometimes assumed to "skip a generation." Fertility Treatments As medical technology made fertility-enhancing treatments more accessible, the twin birth rate has skyrocketed. While not all treatments for infertility increase your odds of fraternal twins, many do, including. Clomid Femara Gonadotropins (also known as injectables) like Gonal-F and Follistim IUI treatment, when used along with fertility drugs IVF treatment Interestingly, while IVF usually results in fraternal twins (due to the transfer of multiple embryos at the same time), some instances of monozygotic twinning can also occur. How to Boost Your Odds of Getting Pregnant With Twins Types of Fraternal Twins Because fraternal twins form from separate eggs and sperm, this can lead to some uncommon circumstances in which the twins have different gestational ages or even different fathers. Superfetation Sometimes hyperovulation happens a few days apart. After one egg is fertilized and begins to travel to the uterus for implantation, another egg is fertilized by sperm from a later incident of sexual intercourse. The result is fraternal twins who are conceived a few days apart. This is known as superfetation. Superfecundation There have even been instances of fraternal twins with different fathers. This occurs when a woman releases multiple eggs and has sexual relations with more than one partner. If an egg is fertilized by sperm from one man, and then another egg is fertilized by sperm from another man, the result is fraternal twins with different fathers. This phenomenon is called superfecundation. Placenta Fusion During pregnancy, the placenta provides vital sustenance to the baby. In a multiple pregnancy with fraternal twins, a placenta develops for each baby. Sometimes, however, the two placentas fuse together and appear to be one single placenta. Because they have their own placentas, fraternal twins are not at risk for some of the conditions that affect monozygotic twins, such as twin-to-twin transfusion syndrome (TTTS) or monoamniotic twins. Causes of Identical and Fraternal Twins Characteristics of Fraternal Twins Verywell / Katie Kerpel Just like other siblings, fraternal twins will share about 50% of their DNA. Each person receives half of their DNA from their mother's egg and the other half from their father's sperm, and so any two offspring will have some overlapping qualities. Monozygotic twins share the same genetic makeup, or genotype (although their DNA is not necessarily identical.) This shared DNA is what makes them often have remarkably similar appearances and characteristics. Unlike monozygotic twins, fraternal twins may look nothing alike, including having different: Eye colorHair colorPersonalitiesStature Or, they may indeed be so similar that they are assumed to be identical, just as some siblings would be remarkably indistinguishable if they were the same age. Twins and multiples are also shaped by their environment after they are born. Some similarities are enhanced because they are raised in the same home, share the same experiences, and are educated in the same schools at the same time. Gender Because fraternal twins originate from separate conceptions, they can both be boys, both girls, or one of each (just like a singleton baby). Chromosomes from the father's sperm determine gender: XX for a girl and XY for a boy. As a result, the chances of fraternal twins resulting in boys, girls, or a combination are the same as for any other pair of siblings. Monozygotic twins, on the other hand, are always the same gender, either two girls or two boys. Fraternal Twins (Dizygotic) Come from their own egg and sperm Share about 50% of DNA May look very different Can be a boy and girl, or the same gender Identical Twins (Monozygotic) Come from a single egg and sperm that splits into two Have same DNA Look the same Are always the same gender Health Risks Twins are affected by your health and habits during pregnancy. Fraternal twins share increased or decreased health risks due to sharing the pregnancy environment. Being pregnant with twins puts additional demands on your body compared with a singleton pregnancy and increases the risk of the following: Anemia Birth defects, including neural tube defects and gastrointestinal and heart abnormalities Gestational diabetes Miscarriage Preeclampsia Pregnancy-induced hypertension Premature birth or preterm labor Risks of a Twin Pregnancy for Mother and Babies Coping While it's not possible to completely eliminate the risks that come with having fraternal twins, you can take some healthy steps to reduce those risks, including: Getting regular prenatal care. Regular prenatal care, ideally with a doctor experienced in twin pregnancies, is essential for catching any problems and receiving proper care. Eating well. Maintaining a healthy diet and weight can help you feel your best during pregnancy and help ensure your babies are born at a healthier weight. Staying hydrated. Dehydration is known to trigger premature labor, which is more likely when you're carrying twins, so keep that water bottle filled. Learning to recognize the signs and symptoms of premature birth. If the signs and symptoms of preterm labor are caught early enough, it may be possible to prevent it so you can carry your pregnancy to term and give your babies a better chance of survival. A Word From Verywell Your fraternal twins will share many things throughout their lives after starting out in the womb together. Enjoy their similarities and differences as they grow. With these facts, you'll be able to tell them how they truly are unique. Having Twins or Multiples? Tips for Staying Positive Was this page helpful? Thanks for your feedback! Get diet and wellness tips to help your kids stay healthy and happy. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit 15 Sources Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Columbia University Department of Obstetrics and Gynecology. Dichorionic twins. Flais SV. The difference between identical and fraternal twins. American Academy of Pediatrics. Updated November 7, 2019. Hoekstra C, Zhao ZZ, Lambalk CB, et al. Dizygotic twinning. Hum Reprod Update. 2008;14(1):37-47. doi:10.1093/humupd/dmm036 Hoekstra C, Willemsen G, Van beijsterveldt CE, Lambalk CB, Montgomery GW, Boomsma DI. Body composition, smoking, and spontaneous dizygotic twinning. Fertil Steril. 2010;93(3):885-93. doi:10.1016/j.fertnstert.2008.10.012 Adashi EY, Gutman R. Delayed childbearing as a growing, previously unrecognized contributor to the national plural birth excess. 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Reprod Fertil Dev. 2013;25(7):1012-1019. doi:10.1071/RD12238 American College of Obstetricians and Gynecologists. Multiple pregnancy. Updated January 2019. National Human Genome Research Institute. Fraternal twins. Chatterjee A, Morison IM. Monozygotic twins: Genes are not the destiny? Bioinformation. 2011;7(7):369-370. doi:10.6026/97320630007369 Fujisawa KK, Todo N, Ando J. Genetic and environmental influences on the development and stability of executive functions in children of preschool age: A longitudinal study of Japanese twins. Infant Child Dev. 2017;26(3):1-27. doi:10.1002/icd.1994 Laine K, Murzakanova G, Sole KB, Pay AD, Heradstveit S, Räisänen S. Prevalence and risk of pre-eclampsia and gestational hypertension in twin pregnancies: A population-based register study. BMJ Open. 2019;9(7):e029908. doi:10.1136/bmjopen-2019-029908