Hello Thanks for the query. What you are describing is known as discordant growth among twins, affecting approximatly 25 % of twin pregnancies. For it to be significant, difference in weight must be atleast 15 %.The causes of discordant fetal growth are not always clear and can sometimes be attributed to more than one factor. Possible causes include blood flow issues, genetic potential, sex of the fetus, environmental factors, and genetic predisposition of the smaller twin to low birth weight that is further complicated by either the crowded growing environment or a placental insufficiency. If the discordance is severe, the smaller twin is severely growth restricted, and has risks of intra uterine death, hence your obstetrician might decide to deliver the twins prematurely. The monitoring is done with serial ultrasounds and colour doppler, which is rightly being done in your case. In case the babies are born premature, steroids are given to enhance their lung maturity. As to what might happen, if the monitoring shows that the situation is not severe enough to threaten one of the twins, your doctor might put you on expectant management with regular monitoring. If she decides to intervene, babies would face complications of preterm birth, more so, the underweight twin, such as growth restriction, underdeveloped lungs, difficulty in breathing, prone to infections, metabolic complications and need for prolonged NICU ( neonatal ICU ) admission Take care.