DISCORDANT FETAL GROWTH

Establishing definitions of IUGR in multiple pregnancies is difficult to determine as the growth rate of twins differ from singleton pregnancies (1). The growth restraining effect of the intrauterine environment becomes evident at about 32 weeks, when the growth of the abdominal circumference (the most accurate ultrasound measurement of fetal mass) of twin gestations begins to slow considerable when compared to singleton pregnancies (1). This effect is even more evident in triplet pregnancies occurring at about 30 weeks and even earlier in quadruplet and quintuplet pregnancies.

Intertwin weight difference (calculated as a percentage of the weight of the larger twin) of 20% or more (other studies have used ranges from 15-40%).

• Weight of smaller twin/weight of larger twin expressed as a percentage (larger twin used as a control).
• Increased perinatal mortality and morbidity due to an increase frequency of malformations, premature labor and growth retardation.

As the weight differences between the twins increase there is an increase in frequency and severity of IUGR in the smaller twin resulting in an increase in perinatal mortality (2). A difference in BPD of 6mm is associated with a 40% incidence of IUGR, and a difference of > 6mm has a 70% incidence (3). A similar relationship between abdominal circumference and frequency and severity of IUGR has been reported (4).

Growth failure in multiple pregnancies is further compounded by (5):

• Higher frequency of maternal and placental complications.
• The incidence of developmental anomalies is almost doubled in multiple pregnancies.
• Variations in placental vascular distribution and intravascular anastomosis in monochorionic monoamniotic twins result in discordant twins.

REFERENCES

1. Socol ML, Tamura RK, Sabbangha RE et.al. Diminished biparietal and abdominal circumference growth in twins. Obstet Gynecol 1984;64:235.
2. Grunbach K, Coleman BG, Arger PH et.al. Twin and singleton growth pattern compared using ultrasound. Radiology 1986;158:237.
3. Houlton MCC. Divergent biparietal growths in twin pregnancies. Obstet Gynecol 1977;49:542.
4. Vintzileos AM, Rodis F. Growth discordance in twins. In: Divon MY (ed). Abnormal Fetal Growth. New York: Elsevier 1991.
5. Manning FA. Intrauterine Growth Retardation. :307-393.