How Common Are Twins?

Patient Presentation
Dizygotic 5-day-old male and female twins came to clinic for their health maintenance visit after discharge from the newborn nursery. They were born to a 24 year old, G2P1 now 3, female at 36 5/7 week gestation by vaginal birth without complications. The mother was attempting to breastfeed each infant every other feed every 2-3 hours. They used formula supplementation after the feeding as needed and used formula for the other infant at each feeding. Her milk was starting to come in at this time. They seemed slightly jaundiced to the parents, but otherwise were doing well. The family history was positive for the mother being a twin herself and her twin sister also had 2 year old twins. There were also maternal cousins who were twins. The father’s family did not have twins in the family.

The pertinent physical exam showed twin infants with weights that were 4% and 6% down from birth weight. The male infant had jaundice to his abdomen and the female had jaundice to the nipple line, but were otherwise well. The laboratory evaluation showed a bilirubin of 12.3 mg/dl for the male and 8.4 mg/dl for the female, both of which were low-risk. The diagnosis of healthy twins was made and the family was to follow up in 5 days or sooner as needed.

Twinning is the conception and development of more than one zygote during one pregnancy. Monozygotic (MZ) twins arise from one zygote that then splits to form two embryos so that the twins are necessarily of the same gender (male-male or female-female). Dizygotic (DZ) twinning arises from the development of two independent zygotes and therefore the genders may be the same or different (male-male, female-female or male-female).

Increased risks of spontaneous DZ twinning includes increased maternal age, parity and gravity, family history including familial clustering, maternal obesity and overweight and smoking. Nutrition itself may or may not play a role. Other factors also cited that are associated with increased rates of twinning are race (e.g. black), ethnicity (e.g. non-Hispanic) and socioeconomic status (e.g. higher). Recently two SNPs were identified which appear to contribute to familial reproductive capacity and DZ twinning (FSHB and SMAD3) in a multi-country, genome-wide association study.

While overall, twins that survive do well with normal outcomes, there can be problems. Maternal complications of twinning include increased stillbirth, neonatal death, premature birth, preeclampsia, post-partum hemorrhage and related problems. Neonatal complications of twinning include preterm birth, intrauterine growth restriction, and discordant growth. Societal costs are also increased for twins. Twins use more health care resources with increased costs than singletons. Some cite information that the cost of raising twins is more than two separate singletons.

Learning Point
While humans have a dominant ovarian follicle selection which usually causes singleton births, twinning is common in humans. Overall in the US, 1 in 30 people is a twin.

Until ~1970 the US rate of overall twin birth was ~1.9% and was constant. The rate has increased to 3.3% in 2009. This appears to be because more DZ twins (particularly opposite sex twins) are being born. Currently, for MZ twins the rate continues to be relatively constant at 3-4 births/1000 around the world. For DZ twins the rate is different with worldwide regional differences. Low rates are found in Asia and Latin America (~1%), but in Africa the rate is much higher at 40 twin births/1000 live births. The Yoruba in Nigeria have a rate of 5%.

The increased rates of DZ are felt to be multifactorial with pharmacological control of fertility, assisted reproductive technologies and increased maternal age being important factors. Note that assisted reproductive technologies also allows options for sperm and egg donations and therefore the DZ twins may be genetic half-siblings.

Questions for Further Discussion
1. Twin studies are common in research, so how would increased DZ twinning possibly affect participant selection for the research studies?
2. Are there differences in congenital anomalies for twins versus singleton deliveries?

Related Cases

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at, the National Guideline Clearinghouse and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for this topic: Twins, Triples and Multiple Births

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images.

To view videos related to this topic check YouTube Videos.

Dawson AL, Tinker SC, Jamieson DJ, Hobbs CA, Rasmussen SA, Reefhuis J; National Birth Defects Prevention Study. Epidemiology of twinning in the National Birth Defects Prevention Study, 1997 to 2007. Birth Defects Res A Clin Mol Teratol. 2015 Feb;103(2):85-99.

Boothroyd C. Twinning: Double, double, toil and trouble? Aust N Z J Obstet Gynaecol. 2016 Oct;56(5):445-446.

Mbarek H, Steinberg S, Nyholt DR, Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility. Am J Hum Genet. 2016 May 5;98(5):898-908.

Rhea SA, Corley RP, Heath AC, Iacono WG, Neale MC, Hewitt JK.
Higher Rates of DZ Twinning in a Twenty-First Century Birth Cohort.
Behav Genet. 2017 Jul 15. (ePub ahead of publication).

Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa