Twin pregnancy brings many exciting moments, but it also comes with regular ultrasound appointments and detailed growth measurements. If your report shows a difference between your babies’ estimated weights, understanding twin pregnancy weight difference can help you know what questions to ask your healthcare provider.

Twin growth discordance describes the percentage difference between the estimated weights of two babies. Although some differences are common, doctors consider each baby’s growth, blood flow, amniotic fluid, and placental arrangement when deciding whether additional monitoring is needed.

Learning how twin pregnancy ultrasound results are interpreted can make these appointments less confusing. The percentage is one part of the assessment, not a diagnosis by itself.

Where the Twin Weight Discordance Percentage Comes From

During a twin pregnancy growth scan, a sonographer measures each baby’s head, abdomen, and thigh bone to calculate the estimated fetal weight twins measurements. The difference between the larger and smaller baby is then calculated as a percentage of the larger baby’s estimated weight.

The formula is: (larger twin’s weight − smaller twin’s weight) ÷ larger twin’s weight × 100. For example, if one baby has an estimated weight of 1,000 grams and the other weighs 820 grams, the difference is 180 grams, resulting in an 18% twin weight difference percentage.

Pro Tip: Keep copies of your ultrasound reports so your healthcare provider can compare measurements across appointments. Estimated fetal weights can vary from actual birth weights, so one scan should not be interpreted in isolation.

Why a Small Weight Difference Between Twins Is Normal

A twin pregnancy weight difference does not automatically indicate a problem because babies can grow at different rates. Fraternal twins have different genetic combinations, and even identical twins may experience differences in growth depending on placental function and how nutrients reach each baby.

Some discordant growth in twins may simply reflect normal variation, particularly when both babies continue to grow along their individual growth curves. Doctors also review growth percentiles, amniotic fluid, and other ultrasound findings before deciding whether the difference needs further investigation.

Pro Tip: Ask whether both babies are growing appropriately for their gestational age rather than judging the pregnancy only by the difference between their estimated weights.

What Doctors Monitor During Twin Pregnancy Growth Scans

During twin pregnancy monitoring, healthcare providers look at the trend in growth discordance rather than relying on a single percentage. A stable difference across several scans may have a different significance from a gap that continues to increase, especially when the smaller baby begins falling behind expected growth patterns.

Your care team may check amniotic fluid, placental function, and blood flow using twin pregnancy Doppler ultrasound. They also determine whether you are carrying monochorionic twins, who share a placenta, or dichorionic twins, who have separate placentas, because placental arrangement influences the recommended scan schedule.

Pro Tip: Ask your clinician whether your twins share a placenta and how that affects your monitoring plan. Your appointment schedule should be based on your pregnancy and the findings from your scans.

Why 20% and 25% Twin Weight Discordance Matter

A 20% weight discordance in twins can lead to closer monitoring under certain clinical guidelines, while 25% discordance in twin pregnancy may prompt specialist assessment when other signs of concern are present. For example, UK NICE guidance recommends at least weekly monitoring when discordance reaches 20% or more, or one baby measures below the 10th percentile, including assessment of blood flow.

Under the same guidance, a weight difference of 25% or more combined with one baby measuring below the 10th percentile calls for referral to a specialist fetal medicine team. These thresholds are clinical guideposts rather than absolute boundaries, and recommendations may differ depending on the country and the individual pregnancy.

Pro Tip: If your report shows a 20% or 25% difference, ask your healthcare provider what the result means in your specific situation and whether additional scans or Doppler assessments are needed.

When Twin Growth Discordance May Indicate Selective Fetal Growth Restriction

Selective fetal growth restriction (sFGR) occurs when one twin is not growing as expected. Twin fetal growth restriction cannot be diagnosed from a weight difference alone; clinicians may also consider estimated fetal weight percentiles, abdominal measurements, and blood flow through the umbilical artery.

A significant gap in discordant growth in twins may raise concern when one baby is particularly small or Doppler findings are abnormal. However, a 25% difference does not automatically mean sFGR if both babies are growing appropriately and their blood flow remains reassuring. Twin-to-twin transfusion syndrome is a separate condition that can affect twins sharing a placenta.

Pro Tip: Ask whether the weight difference is the only concern or whether other signs of growth restriction are present. Your specialist can explain the findings and recommend appropriate follow-up.

What New Research Says About Twin Weight Discordance

Research on twin growth discordance examines how differences in fetal size may relate to premature birth and newborn health. The October 2026 study discussed in the source article examined 434 twin pregnancies involving very early births or very small babies, using ultrasound measurements taken between 20 and 24 weeks to investigate possible links with outcomes.

The study highlighted the importance of gestational age at birth and a weight difference of 25% or more. However, its findings came from a selected high-risk group and should not be applied to every twin pregnancy. The results cannot predict what will happen in an individual pregnancy or determine when twins should be delivered.

Pro Tip: When reading research about twin pregnancy outcomes, check which pregnancies were included in the study and discuss its relevance with your healthcare provider.

Questions to Ask at Your Next Twin Growth Scan

Your next twin pregnancy ultrasound is a useful opportunity to understand how both babies are developing and whether their estimated weight difference is changing. Reviewing your fetal growth scan results with your healthcare provider can help you understand each baby’s growth percentile, blood flow, and whether additional monitoring is recommended.

Consider asking whether the discordance is increasing or decreasing, how each baby is growing individually, whether your twins share a placenta, and when your next scan should take place. You can also ask what findings would change your monitoring plan or lead to a specialist referral.

Pro Tip: Save your questions on your phone before the appointment. This makes it easier to discuss your concerns and understand the next steps in your care.

The Bottom Line on Twin Weight Discordance

Twin weight discordance is one measurement used to assess differences in fetal growth during pregnancy. A modest twin pregnancy weight difference may be common, while a larger or increasing gap can require closer twin pregnancy monitoring, especially if one baby measures small or blood flow is abnormal.

Understanding your twin weight difference percentage is most useful when you consider it alongside individual growth patterns, placental type, and Doppler results. If your report shows 18%, 20%, or 25% discordance, ask your healthcare provider whether the difference is changing and how each baby is growing.

Pro Tip: Do not use the discordance percentage alone to judge your babies’ health or make decisions about delivery. Follow your clinician’s recommendations and contact your maternity team if you have concerns.