Multiple gestation refers to a pregnancy involving two or more fetuses (twins, triplets, or higher-order multiples). This type of pregnancy is automatically designated as high risk from the initial diagnosis. The classification is based on the dramatic physiological burden placed on the mother and the significantly increased risk of complications for the developing fetuses. Carrying multiple babies alters the biological equilibrium compared to a singleton pregnancy, leading to elevated rates of both maternal and fetal morbidity.
Increased Strain on Maternal Systems
The mother’s body must adapt to a greater total fetal and placental mass, demanding a profound increase in blood volume and metabolic activity. This massive expansion of the circulatory system places a significant burden on the heart and kidneys, often contributing to the development of preeclampsia. Women carrying multiples are more than twice as likely to develop this hypertensive disorder, characterized by high blood pressure and potential damage to other organs.
The larger placental size in multiple gestations produces higher levels of pregnancy hormones, which can induce greater insulin resistance and make the mother more susceptible to gestational diabetes. The dramatic increase in blood plasma volume relative to red blood cell mass often leads to hemodilution and anemia, a condition more than twice as common in these pregnancies. This surge in hormones can also exacerbate early pregnancy symptoms, leading to a higher incidence of severe nausea and vomiting known as hyperemesis gravidarum.
Fetal Resource Competition and Growth Restriction
The presence of multiple fetuses and their corresponding placentas within the uterus creates inherent competition for resources. This competition can result in Intrauterine Growth Restriction (IUGR), where one or more fetuses fail to achieve optimal growth potential due to insufficient nutrient and oxygen supply. The risk is particularly pronounced when fetuses share a single placenta, a condition known as monochorionic pregnancy.
In monochorionic pregnancies, the shared placenta often contains vascular connections that lead to an unequal distribution of blood flow, a specialized complication called Twin-to-Twin Transfusion Syndrome (TTTS). This syndrome causes one twin (the donor) to receive too little blood, leading to growth restriction, while the other twin (the recipient) receives too much, straining its cardiovascular system. A related concern is selective IUGR (sIUGR), where significant weight discordance exists between the babies. These resource allocation problems significantly increase the risk for congenital anomalies, which are found at approximately twice the rate in multiple birth babies compared to singletons.
The Pervasive Threat of Preterm Birth
The single greatest factor defining the high-risk status of multiple gestation is the extremely high likelihood of preterm birth. Over 60% of twins and nearly all higher-order multiples are born prematurely, with the average twin delivery occurring around 36 weeks. This early onset of labor is primarily driven by the mechanical stress of uterine overdistension, as the large volume of the fetuses and amniotic fluid physically stretches the uterus to its maximum capacity well before term.
The mechanical stretching of the uterine muscle triggers physiological pathways that initiate labor prematurely. This exposes newborns to life-threatening complications associated with underdeveloped organ systems. Babies born early often require immediate admission to the neonatal intensive care unit (NICU) due to low birth weight and immature lungs.
Consequences of prematurity include respiratory distress syndrome, caused by insufficient surfactant production in the lungs. Very premature babies, those born before 28 weeks, are particularly vulnerable to serious complications such as intraventricular hemorrhage (bleeding into the brain) and chronic feeding difficulties. The earlier the birth, the higher the risk of long-term developmental delays and morbidity, making prematurity the primary driver of poor outcomes.
Delivery and Postpartum Complications
The process of labor and delivery itself carries increased risks for both the mother and the babies. The rate of Cesarean section is significantly higher, even in cases planned for vaginal delivery, due to potential complications like fetal distress or malpresentation, such as one twin being breech.
Following the delivery, the mother faces a substantially elevated risk of Postpartum Hemorrhage (PPH). The uterine muscle, stretched far beyond its normal capacity, can become fatigued and fail to contract effectively after birth, a condition called uterine atony. This failure prevents the blood vessels that supplied the placentas from closing, leading to excessive bleeding, a risk compounded by the multiple large placental sites that must detach.