Multiple Pregnancy occurs when two or more fetuses develop during the same pregnancy. Twins are the most common form, while triplets and higher-order pregnancies are less common. Multiple Pregnancy can result from the fertilisation of more than one egg, producing genetically distinct siblings, or from division of a single fertilised embryo, producing genetically highly similar monozygotic siblings.
The most clinically important classification is often chorionicity and amnionicity, which describe the number of placentas and amniotic sacs rather than simply whether twins are identical. Dichorionic twins have two chorionic compartments and generally have separate placental circulations, while monochorionic twins share a placenta and therefore have additional risks arising from vascular connections between fetal circulations. Diamniotic twins have separate amniotic sacs, whereas monoamniotic twins share one sac. Accurate early ultrasound determination of chorionicity is important because surveillance and risk differ substantially between these configurations.
Multiple Pregnancy is more likely with increasing maternal age, previous pregnancies and certain fertility treatments, particularly treatments that result in more than one embryo or increase the likelihood of multiple ovulation. Assisted reproductive technology has changed the epidemiology of Multiple Pregnancy, although policies encouraging single-embryo transfer have reduced multiple births associated with fertility treatment in many settings.
Dizygotic twinning is influenced by factors affecting multiple ovulation, while monozygotic twinning appears largely unpredictable and is not strongly controlled by ordinary lifestyle factors. The number of placentas does not always correspond intuitively to the number of fetuses because a single embryo can divide after implantation and produce a shared placenta. Ultrasound in early pregnancy is therefore important for establishing chorionicity before the anatomy becomes more difficult to interpret. This classification becomes particularly important when twins share a placenta because complications can arise from abnormal inter-twin vascular connections.
Multiple Pregnancy increases maternal and fetal demands and is associated with higher rates of complications than singleton pregnancy. Premature Birth is particularly common because the uterus becomes increasingly distended and placental and fetal demands rise. Maternal complications can include anaemia, hypertensive disorders, gestational diabetes, excessive nausea, postpartum haemorrhage and operative delivery. Fetal complications include growth restriction, discordant growth, congenital abnormalities, malpresentation and complications specific to shared placental circulation. The degree of risk varies considerably according to chorionicity and amnionicity.
Dichorionic twins generally have fewer complications related to placental sharing than monochorionic twins. Monoamniotic twins have additional risks because both fetuses share one amniotic cavity, increasing the potential for umbilical-cord entanglement. Higher-order pregnancies can carry substantially greater risks and usually require specialist maternal-fetal medicine involvement.
Surveillance in Multiple Pregnancy is more intensive because growth and wellbeing can change differently between fetuses. Ultrasound is used to assess fetal growth, amniotic fluid, placental relationships and blood-flow patterns. In monochorionic pregnancies, clinicians specifically monitor for twin-to-twin transfusion syndrome and other complications arising from shared placental circulation.
Twin-to-twin transfusion syndrome occurs when vascular connections within a shared placenta produce an unbalanced net transfer of blood between fetuses, resulting in a donor fetus with reduced circulating volume and a recipient fetus with excessive volume. Twin anaemia-polycythaemia sequence represents another imbalance in blood exchange and can develop through small placental vascular connections. Selective fetal growth restriction can affect one fetus more severely than the other. The surveillance schedule depends on chorionicity and local specialist protocols, with monochorionic pregnancies generally requiring more frequent assessment than dichorionic pregnancies.
Management of Multiple Pregnancy begins before birth with nutritional, medical and obstetric care tailored to increased physiological demands. Folic acid and iron requirements may require particular attention, especially when anaemia develops. Blood pressure and glucose are monitored because hypertensive disorders and gestational diabetes are more common. Fetal surveillance becomes progressively more important as pregnancy advances. Delivery planning depends on gestational age, chorionicity, fetal presentation, growth and maternal or fetal complications.
Vaginal birth can be possible for selected twin pregnancies when the leading fetus is appropriately positioned and no contraindication exists, while caesarean birth may be recommended for particular presentations or complications. The birth of twins also requires coordinated management because after delivery of the first fetus, the position and condition of the second can change. Continuous fetal monitoring and an experienced obstetric and neonatal team may therefore be appropriate.
Multiple Pregnancy creates special intrapartum and postpartum considerations. Labour may be complicated by malpresentation, dysfunctional contractions or increased risk of postpartum haemorrhage because the uterus is more distended. After the first baby is born, clinicians assess the second baby's position and heart rate and determine whether continued vaginal birth, assisted delivery or caesarean birth is safest. The interval between births can vary and is not inherently problematic when maternal and fetal conditions remain reassuring.
Newborns are more likely to require neonatal assessment because prematurity, low birth weight, respiratory difficulties and feeding problems are more common. After delivery, maternal recovery can be affected by blood loss, anaemia and the physical demands of caring for more than one newborn. Feeding, sleep and practical support can also be more challenging, particularly when one or both babies require neonatal care.
Multiple Pregnancy is not synonymous with identical twins. Identical twins can have different placental arrangements depending on when the embryo divides, while non-identical twins generally have separate placental compartments but rare placental fusion can make visual appearance misleading. Ultrasound determination of chorionicity therefore matters more clinically than assumptions about genetic identity. Another common misconception is that all twin pregnancies carry the same level of risk; the distinction between dichorionic and monochorionic pregnancies is fundamental.
Shared placental circulation creates complications that cannot occur in the same way in pregnancies where each fetus has an independent placenta. Likewise, a multiple pregnancy is not automatically an indication for caesarean birth. Mode and timing of delivery depend on the specific pregnancy rather than the mere presence of two fetuses.
The history of Multiple Pregnancy is intertwined with advances in ultrasound, fertility treatment and neonatal care. Improved early imaging allows clinicians to identify chorionicity, fetal number and major structural abnormalities, while neonatal advances have improved outcomes for premature multiples. Nevertheless, the central challenge remains balancing the risks of continuing pregnancy against the risks of Premature Birth. Early specialist involvement can identify complications such as shared-placenta transfusion disorders, growth restriction and maternal hypertension.
The pregnancy also illustrates how placental architecture can be as important as fetal genetics in determining clinical risk. A comprehensive understanding therefore requires simultaneous attention to embryo division, placental number, amniotic sacs, fetal growth, maternal physiology and delivery planning rather than treating twins or higher-order pregnancies as a single uniform category.
