During a pregnancy with twins or other multiples, it is normal for the babies to vary in size slightly. When this growth disparity gets too large, the smaller fetus could become endangered. Discordant fetal growth between babies sharing a womb may be the sign of another problem or it may occur on its own.
Discordant fetal growth occurs when, during a pregnancy in which the mother is carrying more than one baby, one fetus is significantly larger or smaller than the other or others. While many physicians and doctors disagree on the precise disparity in growth required to diagnose discordancy, it is typically described as a difference of 15 to 25 percent or more between the weights of the babies, according to The Fetus.net.
Discordant fetal growth may occur in identical twin pregnancies, in which the babies have the same genes and share an amniotic sac, or in fraternal twins, which have the genetics of siblings. According to The Fetus.net, between 15 and 29 percent of twin pregnancies develop discordant fetal growth and in 3 percent of the affected twin sets, the discrepancy in weight at birth is over 1,000 grams.
While in some cases the cause of discordant fetal growth is unknown, intrauterine growth restriction, or IUGR, in the case of one fetus may be a factor. IUGR occurs when one baby does not get the nutrients it needs, due to problems with the placenta or umbilical cord, the womb environment or genetic factors. In some pregnancies, a condition called twin-to-twin transfusion occurs in which the larger baby gets the majority of the nutrients due to a defect in the placenta that causes mixing of the blood vessels. Chromosomal disorders in one fetus but not the other may cause a discrepancy in size, but this is usually considered a separate problem, not discordant fetal growth.
Detecting discordant fetal growth is difficult early in pregnancy, but may be identified using ultrasound as the pregnancy progresses. One sign of uneven growth is if one baby measures at a later gestational age than the other baby or babies in the womb. If discordant fetal growth is discovered, additional tests are generally given to determine the cause. These may include amniocentesis or chorionic villus sampling to look for chromosomal abnormalities, blood tests of the mother or additional high-level ultrasounds.
Babies with severe discordant fetal growth, over a 25 percent discrepancy, are more likely to die in utero, be stillborn or have severe complications after birth. According to Advances in Neonatal Care, the larger twin in a discordant growth pair may have a higher risk of acute respiratory difficulty after birth, but have fewer complications overall. Twin or multiples pregnancies in which the two fetuses differ significantly in size may be born prematurely or may require an induction or Cesarean section to remove the babies from the womb before natural labor begins.