Twin Growth Assessment: Chorionicity, Discordance and Plotting
Twin pregnancies carry a higher risk of growth problems than singletons. Growth surveillance in twins depends on three things: knowing the chorionicity, plotting each twin consistently, and comparing the two twins with each other.
1. Chorionicity comes first
Chorionicity (whether the twins share a placenta) is the most important factor for risk in a twin pregnancy and decides how often scans are done. It is best determined in the first trimester (lambda or T-sign).
| Type | Placenta / sac | Growth issues to watch |
|---|---|---|
| DCDA (dichorionic diamniotic) | Two placentas, two sacs | Selective growth restriction, discordant growth |
| MCDA (monochorionic diamniotic) | Shared placenta, two sacs | Selective FGR, twin–twin transfusion syndrome, TAPS |
| MCMA (monochorionic monoamniotic) | Shared placenta, one sac | As MCDA, plus cord entanglement |
The tool records chorionicity and prints it on the report, together with each twin's maternal side, order and presentation, to help identify the same twin consistently at each scan.
2. Label the twins consistently
Serial growth assessment only works if "Twin A" at one scan is the same fetus as "Twin A" at the next. Use a stable description, such as maternal side (right or left) and position relative to the cervix (leading or non-leading), and record it every time.
3. Inter-twin EFW discordance
Discordance compares the estimated fetal weights of the two twins at the same scan:
For example, if Twin A is 1,800 g and Twin B is 1,450 g: (1,800 − 1,450) ÷ 1,800 × 100 = 19.4%.
The tool calculates discordance automatically for every visit and shows it in the Measurement History and on the printout. ISUOG guidance on twin pregnancy recommends referral to a fetal medicine centre when EFW discordance is 25% or more, while many units start closer surveillance at lower levels. Follow your local protocol.
Discordance is a ratio of two estimates, each with its own error (often 10–15%). A single discordance value should be interpreted with the individual growth curves and Doppler findings.
4. Plotting each twin separately
On this site, Twin A and Twin B have their own charts for each parameter, plotted against the WHO reference. This shows whether one twin is falling away from its own previous trajectory, even when the discordance is still small.
The WHO reference was built from singleton pregnancies. There is ongoing debate about whether singleton charts or twin-specific charts should be used to assess twins. Twin-specific charts reflect the slower growth of twins in the third trimester. Singleton charts keep the same standard of "normal" for every fetus. Use the reference your unit has agreed on, and interpret twin growth with that in mind.
5. What the growth chart does not show
- Twin–twin transfusion syndrome (TTTS) is diagnosed mainly from the amniotic fluid difference (polyhydramnios in one sac, oligohydramnios in the other) in monochorionic twins, not from growth. The tool records DVP for each twin, but staging needs specialist assessment.
- TAPS (twin anaemia–polycythaemia sequence) is diagnosed with middle cerebral artery Doppler, which this tool does not record.
- Selective FGR in monochorionic twins is classified with umbilical artery Doppler patterns. Record the Doppler findings, but classification needs specialist review.
References
- Khalil A, Rodgers M, Baschat A, et al. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy. Ultrasound Obstet Gynecol. 2016;47(2):247–263.
- Kiserud T, Piaggio G, Carroli G, et al. The World Health Organization Fetal Growth Charts. PLOS Med. 2017;14(1):e1002220. doi:10.1371/journal.pmed.1002220