Twin growth comparator
Compare twins’ estimated weights side by side. See the relative difference, understand limits, and discuss growth discordance with your specialist.
Twin growth comparator
Compare the estimated weights of Twin A and Twin B side by side with this free twin growth comparator. The result can help you prepare questions for your care team, but it cannot determine whether either baby is growing normally.
What a twin growth comparator shows
A twin growth comparator places the estimated fetal weights for each twin next to each other. It shows which estimate is larger and expresses the difference relative to the larger estimate, often described as estimated weight discordance.
The result describes the relationship between the estimates entered. It does not show whether either baby is small or large for gestational age, whether growth has slowed, or whether a pregnancy needs treatment.
Those questions require the complete ultrasound report and clinical context. A maternal-fetal medicine specialist or obstetric clinician may consider each baby’s growth trend, measurements, fluid, blood-flow studies, placental arrangement, and other findings.
How to compare the estimated weights
- Find the estimated fetal weight for Twin A on the ultrasound report.
- Find the estimated fetal weight for Twin B from the same examination.
- Enter both estimates in the units requested by the interactive widget on this page.
- Check that the twins were not accidentally reversed and that no value was copied from an earlier scan.
- Review the side-by-side result and save the ultrasound report for your next appointment.
Use estimates from the same ultrasound visit whenever possible. Comparing measurements taken at different gestational ages can make the gap look larger or smaller because fetal growth continues between examinations.
Use the pregnancy dating already established by your clinician. By convention, an estimated due date is based on a pregnancy length of 280 days, or 40 weeks, from the first day of the last menstrual period when that method is appropriate (ACOG, 2017).
Twin weight comparison at a glance
| Comparator item | What it means | What it does not tell you |
|---|---|---|
| Twin A estimated weight | The ultrasound-based estimate entered for the baby labeled Twin A | The baby’s exact current weight |
| Twin B estimated weight | The ultrasound-based estimate entered for the baby labeled Twin B | The baby’s exact current weight |
| Larger estimate | Which entered weight is higher | Whether that baby is growing appropriately |
| Relative difference | The gap between the estimates relative to the larger estimate | A diagnosis or prediction of outcome |
| Scan context | Gestational age, measurement date, growth trend, and other report findings | Information the comparator can calculate from weights alone |
Labels should stay consistent across your records. If a report changes how the babies are labeled or you are unsure which estimate belongs to which twin, ask the ultrasound department or clinician before comparing results.
How to interpret estimated weight discordance
Discordance means there is a relative difference between the estimated weights. A larger result does not, by itself, explain why the estimates differ or establish that one twin has a growth problem.
Interpretation depends on details that are not captured by a simple comparator. These include whether the twins share a placenta, whether either estimate has crossed growth-chart lines, and whether fluid or blood-flow findings are concerning.
No sourced clinical cutoff for normal or abnormal twin discordance was included in the evidence supplied for this page, so this tool does not label a result safe, unsafe, mild, or severe. Ask your specialist what the result means for your particular twin pregnancy and whether follow-up imaging is recommended.
A trend can be more informative than an isolated comparison, but earlier results should remain attached to their scan dates. Do not treat estimates from separate examinations as though they were collected at the same point in pregnancy.
Limits of ultrasound weight comparisons
An ultrasound weight is an estimate calculated from fetal measurements rather than a direct weighing. The comparator therefore compares estimated values, not the babies’ exact weights.
Small differences in image measurement or fetal position can affect the values entered into the calculation. This page does not have a sourced figure for the expected error of an ultrasound weight estimate, so it does not apply an accuracy adjustment or display an uncertainty range.
The tool also cannot evaluate anatomy, placental function, amniotic fluid, blood flow, fetal movement, or the pattern of growth over time. It cannot diagnose fetal growth restriction, twin-to-twin transfusion syndrome, or another pregnancy complication.
Contact your maternity unit promptly if movement is reduced or the usual movement pattern changes. There is no set number of movements that applies to every baby; becoming familiar with the usual pattern is the important point in NHS guidance (NHS, 2020). Do not wait for a growth-comparator result before seeking help about a movement change or another urgent concern.
Questions to take to your twin specialist
Bring the complete ultrasound report rather than only the comparator result. The report gives your clinician the measurements, gestational age, fluid findings, and other information needed to interpret estimated weights.
- Are both estimated weights appropriate for this gestational age?
- How does each baby’s growth compare with the previous scan?
- Which growth chart or reference is being used for this pregnancy?
- Does the placental arrangement affect how you interpret the difference?
- Were fluid levels and blood-flow studies reassuring?
- When should growth be checked again?
- Which symptoms or movement changes should prompt an immediate call?
Your clinician may be comfortable with a difference that looks worrying without context, or may want closer follow-up even when the estimates appear similar. The plan should come from the full clinical picture, not the calculator alone.
Frequently asked questions
What is a twin growth comparator?
A twin growth comparator places both estimated fetal weights side by side and shows their relative difference. It is a simple way to organize ultrasound values before discussing them with your care team. It does not determine whether either twin is growing normally because it does not include growth charts, earlier scans, placental information, fluid, or blood-flow findings.
How do you calculate twin growth discordance?
Twin growth discordance compares the gap between the smaller and larger estimated weights with the larger estimate. The widget performs that comparison after you enter both ultrasound values. Check that both estimates come from the same scan and use the requested units. Your specialist should interpret the result because the calculation alone is not a diagnosis.
What is a normal weight difference between twins?
There is no cutoff provided by the sourced evidence for this page that can classify a twin weight difference as normal or abnormal. Meaning depends on gestational age, growth trends, placental arrangement, fluid, blood-flow studies, and other findings. Ask your obstetric clinician or maternal-fetal medicine specialist how the difference should be interpreted in your pregnancy.
Can twins be different sizes and still be healthy?
Yes, different estimated sizes do not automatically mean that either twin is unhealthy. The significance depends on how each baby has been growing and what the rest of the ultrasound shows. Because ultrasound weights are estimates, a visible gap also has measurement uncertainty. Your clinician can explain whether the pattern needs routine monitoring or closer follow-up.
Does a twin growth comparator diagnose fetal growth restriction?
No, a weight comparison cannot diagnose fetal growth restriction. Diagnosis and management require clinical assessment, appropriate growth references, earlier measurements, and often additional ultrasound findings. This comparator does not assess anatomy, fluid, blood flow, placental function, or fetal well-being. Use its result as a discussion aid rather than a medical conclusion.
Which ultrasound weights should I enter?
Enter the estimated fetal weight for each twin from the same ultrasound examination. Do not substitute an individual head, abdomen, or leg measurement for the report’s estimated weight. Keep the Twin A and Twin B labels consistent. If the labels appear to have changed or a value is unclear, confirm them with the ultrasound department.
How accurate is a twin ultrasound weight estimate?
An ultrasound weight is an estimate derived from fetal measurements, not a direct weighing. Image quality, fetal position, and measurement variation can affect the result. We do not have a sourced accuracy figure in the supplied evidence, so the comparator does not claim a specific margin of error. Your specialist can interpret the estimates alongside the full scan.
Understanding twin growth differences
This comparator is a screening aid, not a diagnosis or delivery-timing tool. EFW discordance is calculated as (larger twin EFW − smaller twin EFW) ÷ larger twin EFW × 100. Interpretation also depends on chorionicity, centiles, Doppler results, maternal health and the full scan.
| EFW difference | Guideline context |
|---|---|
| <20% | Mild size difference |
| 20-24% | Threshold where monitoring is often increased |
| ≥25% | May prompt specialist fetal medicine review |
NICE twin and triplet pregnancy guidance recommends increased monitoring at ≥20% discordance or when either twin is below the 10th centile.