What is the twin discordance formula?

NICE NG137 recommendation 1.4.20 gives it as the estimated fetal weight of the larger baby minus the smaller, divided by the larger, multiplied by 100. So 2000g and 1600g is a 20% discordance, because the 400g difference is a fifth of 2000g. Dividing by the smaller baby, or by the average, gives a different and incorrect figure.

Is 20% discordance between twins serious?

It is the point at which NICE recommends stepping monitoring up to at least weekly, with Doppler assessment of umbilical artery flow for each baby. That is a change in how closely you are watched rather than a diagnosis. Plenty of twins sit above 20% and are entirely well.

NICE recommends referral to a tertiary level fetal medicine centre when discordance reaches 25% or more and either baby is also below the 10th centile for gestational age, because that combination is a clinically important indicator of selective fetal growth restriction. The referral is for specialist assessment.

How much difference between twins is normal?

Some difference is usual and expected, particularly for dichorionic twins with separate placentas, where the two babies simply have different growth potential. Most twin pairs sit well below 20%. A difference on its own, without a low centile or abnormal Doppler, is watched rather than treated.

How accurate is estimated fetal weight?

Less accurate than people assume. Ultrasound estimates commonly carry an error margin of around 10% either way, which means a discordance figure close to a threshold could sit on either side of it. This is one reason your team looks at the trend across several scans rather than reacting to a single number.

Does this work for birth weights too?

Yes, the arithmetic is the same and birth weight discordance uses the same formula. It is a measured number rather than an estimate, so there is no ultrasound error margin to allow for.

NICE gives a separate calculation for trichorionic triplets: largest against smallest, and largest against middle, each as its own percentage. You can run this tool twice to get both figures.

Twin weight discordance calculator (NICE 20% and 25%)

Twin weight discordance calculator

Baby A weight

Baby B weight

Unit

Worked out in your browser. Nothing you enter is sent anywhere. This is general information, not medical advice.

What the number is

Discordance is the gap between your two babies expressed as a share of the bigger one. The reason it is defined that way, rather than as a raw weight difference, is that 400g between two babies means something very different at 1.6kg than it does at 3.4kg.

NICE sets the formula out explicitly in guideline NG137: take the larger estimated fetal weight, subtract the smaller, divide by the larger, multiply by 100. Two babies at 2000g and 1600g are 20% discordant. The same 400g at 3000g and 2600g is 13%.

Why 20 and 25 are the numbers

These are not arbitrary. NG137 uses 20% as the point to increase monitoring to at least weekly with umbilical artery Doppler for each baby, and 25% combined with either baby below the 10th centile as the point to refer to a tertiary fetal medicine centre, because that pairing is an indicator of selective fetal growth restriction.

Both are thresholds for how closely you are watched, not predictions. A pair above 20% with normal Dopplers and both babies on healthy centiles is a different situation from a pair at the same percentage where one baby is small for dates, which is exactly why the second threshold has two conditions rather than one.

The part people miss

An estimated fetal weight from a scan is an estimate. The error margin is commonly around 10% in either direction, and it applies to both babies independently, so a discordance figure sitting a point or two either side of a threshold is inside the noise.

This is why your team watches the trend. Three scans showing discordance widening from 12% to 18% to 24% tells them something a single reading of 21% does not. If you are tracking this yourself, record the whole series rather than the latest number.

If you are reading this worried

A size difference between twins is common, it is usually monitored rather than acted on, and the great majority of discordant pairs are born well. What this tool can do is tell you what the number is and what the guideline says about it, so that the conversation with your midwife starts from the same figure they are looking at. What it cannot do is tell you what it means for your pregnancy. Bring it to them.

If the number has gone up since your last scan, or you are at 25% or more, contact your maternity team rather than waiting for the next routine appointment.

Quick answer

Twin weight discordance is the difference between the two babies as a percentage of the larger one. NICE guideline NG137 defines it as (larger minus smaller, divided by larger, times 100) and sets two thresholds: at 20% or more it recommends monitoring at least weekly including umbilical artery Doppler, and at 25% or more, with either baby below the 10th centile, it recommends referral to a tertiary fetal medicine centre. This calculates the percentage and tells you which side of those numbers you are on. It is not a diagnosis and it does not replace your scan report.

How to use it

Enter both weights. Use estimated fetal weights from the same scan, or both birth weights. Same unit for both; the calculator does not convert between them.
Read the percentage. You get the discordance figure and whether it sits below 20%, at 20% or more, or at 25% or more.
Take the number to your team. The thresholds decide monitoring, not outcome. Your midwife or obstetrician reads it alongside centiles, Doppler and chorionicity.

Frequently asked questions

What is the twin discordance formula?: NICE NG137 recommendation 1.4.20 gives it as the estimated fetal weight of the larger baby minus the smaller, divided by the larger, multiplied by 100. So 2000g and 1600g is a 20% discordance, because the 400g difference is a fifth of 2000g. Dividing by the smaller baby, or by the average, gives a different and incorrect figure.
Is 20% discordance between twins serious?: It is the point at which NICE recommends stepping monitoring up to at least weekly, with Doppler assessment of umbilical artery flow for each baby. That is a change in how closely you are watched rather than a diagnosis. Plenty of twins sit above 20% and are entirely well.
What happens at 25%?: NICE recommends referral to a tertiary level fetal medicine centre when discordance reaches 25% or more and either baby is also below the 10th centile for gestational age, because that combination is a clinically important indicator of selective fetal growth restriction. The referral is for specialist assessment.
How much difference between twins is normal?: Some difference is usual and expected, particularly for dichorionic twins with separate placentas, where the two babies simply have different growth potential. Most twin pairs sit well below 20%. A difference on its own, without a low centile or abnormal Doppler, is watched rather than treated.
How accurate is estimated fetal weight?: Less accurate than people assume. Ultrasound estimates commonly carry an error margin of around 10% either way, which means a discordance figure close to a threshold could sit on either side of it. This is one reason your team looks at the trend across several scans rather than reacting to a single number.
Does this work for birth weights too?: Yes, the arithmetic is the same and birth weight discordance uses the same formula. It is a measured number rather than an estimate, so there is no ultrasound error margin to allow for.
What about triplets?: NICE gives a separate calculation for trichorionic triplets: largest against smallest, and largest against middle, each as its own percentage. You can run this tool twice to get both figures.