Illustration of twin babies with a slight size difference, representing birth weight discordance.

Twin Birth Weight Discordance: How Much Difference is Too Much?

Mira Elwood in Health & Wellness December 2025 • 4 min read.

"A new study reveals the degree of birth weight difference in twins that significantly raises the risk of adverse neonatal outcomes, offering clarity for parents and healthcare providers."

Twin pregnancies are often filled with unique joys and challenges, and one common concern is the difference in birth weights between the twins, known as birth weight discordance. While some difference is normal, a significant disparity can lead to increased risks. A recent study published in the American Journal of Perinatology sheds light on this issue, providing valuable insights for expectant parents and healthcare professionals.

The study, led by Lena H. Kim, MD, and colleagues, aimed to determine the degree of twin birth weight discordance associated with adverse neonatal outcomes. By analyzing a large cohort of twin deliveries, the researchers identified a critical threshold that indicates a higher risk of complications. This information is crucial for making informed decisions about prenatal care and delivery management.

This article will explore the key findings of the study, explaining what constitutes a significant birth weight discordance and the potential implications for the health of newborns. Understanding these risks can empower parents to engage in proactive discussions with their healthcare providers and ensure the best possible outcomes for their twins.

What Level of Twin Birth Weight Discordance Raises Concern?

Illustration of twin babies with a slight size difference, representing birth weight discordance.

The study defined birth weight discordance as the percentage difference between the larger and smaller twin, calculated as [(larger twin weight - smaller twin weight) / larger twin weight] x 100. The researchers analyzed data from 27,276 twin deliveries in the United States between 2012 and 2014, focusing on vaginal deliveries of twins with the first twin in a head-down position, delivered between 36 and 40 weeks of gestation.

The primary outcome the researchers looked at was a composite measure of neonatal morbidity, which included any of the following:

A 5-minute Apgar score less than 7 (indicating potential difficulties in transitioning to life outside the womb).
Admission to a neonatal intensive care unit (NICU).
Need for mechanical ventilation for more than 6 hours.
Neonatal seizures.
Neonatal transport to a higher level of care facility.

The results indicated that a birth weight discordance of more than 20% was associated with a significantly increased risk of this composite adverse neonatal outcome. Specifically, the adjusted odds ratios (aORs) for different levels of discordance compared to a discordance of less than 20% were:

Empowering Parents with Knowledge

The findings of this study underscore the importance of monitoring twin birth weight discordance during pregnancy. By understanding the risks associated with different degrees of discordance, healthcare providers can tailor prenatal care and delivery management to optimize outcomes for both twins. For expectant parents, this knowledge empowers them to engage in informed discussions with their medical team, ask pertinent questions, and actively participate in decisions affecting their babies' health. While a twin birth weight discordance greater than 20% does mean increased odds of adverse neonatal outcomes, appropriate surveillance with nonstress tests, Doppler ultrasound, and timing of delivery may help ameliorate the morbidity seen in the high-risk subgroup.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1055/s-0038-1675769, Alternate LINK

Title: Association Between The Degree Of Twin Birthweight Discordance And Perinatal Outcomes

Subject: Obstetrics and Gynecology

Journal: American Journal of Perinatology

Publisher: Georg Thieme Verlag KG

Authors: Lena H. Kim, Aaron B. Caughey, Lynn M. Yee, Yvonne W. Cheng

Published: 2018-11-26

Everything You Need To Know

What is twin birth weight discordance?

Birth weight discordance refers to the difference in birth weights between twins. It's calculated as a percentage: [(larger twin weight - smaller twin weight) / larger twin weight] x 100. While some difference is normal, significant discordance can indicate higher risks for the newborns. The article highlights the importance of monitoring this difference during twin pregnancies.

What level of twin birth weight discordance is considered concerning?

The study, led by Lena H. Kim, MD, and colleagues, found that a birth weight discordance of more than 20% is associated with a significantly increased risk of adverse neonatal outcomes. This threshold is a crucial finding as it helps healthcare providers and parents assess potential risks and make informed decisions about prenatal care and delivery management.

What are the adverse neonatal outcomes associated with birth weight discordance?

Adverse neonatal outcomes, as defined in the study, include a range of complications. These include a 5-minute Apgar score less than 7, indicating potential difficulties in transitioning to life outside the womb; admission to a neonatal intensive care unit (NICU); the need for mechanical ventilation for more than 6 hours; neonatal seizures; and neonatal transport to a higher level of care facility. The study used a composite measure, meaning the presence of any of these factors contributed to a negative outcome.

Why is understanding birth weight discordance important for parents?

Understanding the risks associated with twin birth weight discordance empowers parents to engage in informed discussions with their medical teams. They can ask pertinent questions and actively participate in decisions affecting their babies' health. This knowledge allows for proactive prenatal care and delivery management strategies, potentially improving outcomes for both twins. Appropriate surveillance with nonstress tests, Doppler ultrasound, and timing of delivery may help.

What are the implications of twin birth weight discordance?

The implications of birth weight discordance are significant because it's directly linked to potential complications for the twins. A discordance greater than 20% signals a higher risk of adverse neonatal outcomes, emphasizing the need for careful monitoring and tailored medical interventions. This also allows for better preparedness from healthcare providers and parents to handle any potential issues that may arise during or after birth.