Is it safe? · Pregnancy Smart
Selective Fetal Growth Restriction in Twins: Next Steps
What does the evidence say about selective fetal growth restriction in twins?
- A size difference alone does not capture every case of selective growth restriction. ISUOG discusses several assessment criteria, including a very small estimated fetal weight, discordance, and abnormal umbilical artery resistance. Ask which findings support the conclusion in your pregnancy. 1
- Unequal sharing of placental tissue and circulation can contribute to selective growth restriction in monochorionic twins. Other findings may coexist. The team reviews anatomy, cord attachment, fluid, and blood-flow information rather than assuming a single cause from the weight difference. 2
- Doppler evaluates circulation in vessels such as the umbilical artery and middle cerebral artery. It helps distinguish different risk patterns and track change. A growth estimate and a blood-flow result are complementary parts of the assessment. 3
- Not every difference in twin size indicates a serious complication. The degree, trend, and surrounding findings matter. More frequent scans may be needed when growth restriction is suspected, and delivery planning considers the health of both babies. 4
- Earlier birth can reduce time exposed to a worsening pregnancy complication but adds risks related to prematurity. A specialist discussion should explain how the team weighs the smaller twin's condition against the consequences of early delivery for both babies. 5
How is selective growth restriction followed in twins?
- First trimester. Early ultrasound should document dating, placental arrangement, and consistent labels for the twins.
- Second trimester. During growth follow-up, clarify the criteria used and circulation findings for each twin.
- Third trimester. When discussing birth, ask about the balance of fetal risk and prematurity for both babies.
Questions to discuss with your care team
| Decision point | What to clarify |
|---|---|
| Classification | Which size and Doppler findings are concerning? |
| Surveillance | Which measurements will be repeated and how often? |
| Decision threshold | What change would lead to a different plan or earlier birth? |
Frequently asked questions
Is every difference in twin weight called selective growth restriction?
No. Specialists consider the size percentile, degree of discordance, Doppler findings, and other criteria. Ask whether the report describes an isolated size difference, a small baby, or a pattern meeting criteria for growth restriction.
How is weight discordance calculated?
It is the difference between the larger and smaller estimated weights divided by the larger estimated weight, expressed as a percentage. Ultrasound weights are estimates, so the number still needs clinical context.
Why does sharing a placenta change the discussion?
Shared placental tissue and vascular connections can link the twins' circulation and risks. Separate placentas create a different clinical situation. The same weight difference can therefore lead to different surveillance and counseling.
What do types I, II, and III mean in monochorionic twins?
They describe patterns of end-diastolic blood flow in the smaller twin's umbilical artery: positive, persistently absent or reversed, or intermittently absent or reversed. The classification informs counseling but does not replace the complete assessment.
Why are Doppler scans repeated between growth measurements?
Blood-flow information can change and provides a different view of fetal condition from size alone. Ask what vessels are being followed and which finding would lead to a change in monitoring or delivery planning.
Can selective growth restriction occur with TTTS?
Yes. Complications of a shared placenta can coexist. The team checks fluid and other features as well as growth to determine which conditions are present and how they affect the plan.
Does a smaller twin always need immediate delivery?
No. Timing balances the condition of the pregnancy with the effects of early birth on both babies. Ask what findings support continued surveillance and what would make earlier delivery preferable.
What should I bring to a specialist review?
Bring prior ultrasound reports and the current care plan so the team can compare each baby's growth and other findings over time. Ask for a clear explanation of the monitoring schedule and how to report new concerns.
Related in the library
Is it safe?
Monochorionic Diamniotic Twins: What MCDA Means
Is it safe?
Dichorionic Diamniotic Twins: What DCDA Means
Comparisons
Vaginal Birth vs Cesarean for Twins: Planning the Decision
Is it safe?
Twin-Twin Transfusion Syndrome: Findings and Next Steps
Is it safe?
Twin Anemia-Polycythemia Sequence: Understanding TAPS
Symptoms
Fetal growth restriction
Is it safe?
Twin Reversed Arterial Perfusion: Understanding TRAP
References
ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy
PubMed Central · https://pmc.ncbi.nlm.nih.gov/articles/PMC11788470/
Surveillance of Monochorionic Twins
ISUOG · https://www.isuog.org/education/visuog/obstetrics/multiple-pregnancy/surveillance-of-monochorionic-twins.html
Doppler assessment in twin pregnancies
ISUOG · https://www.isuog.org/clinical-resources/patient-information-series/patient-information-pregnancy-conditions/multiple-pregnancies/doppler-assessment-in-twin-pregnancies.html
ACOG · https://www.acog.org/womens-health/faqs/multiple-pregnancy
ACOG · https://www.acog.org/womens-health/faqs/preterm-labor-and-birth
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.