Every twin pregnancy involves at least two umbilical cords, but how those cords relate to each other and to the placenta depends entirely on the type of twins. Fraternal twins and some identical twins each develop their own placenta and their own cord, functioning independently the way any singleton’s cord would. Identical twins who share a single placenta, however, are connected to the same blood supply, and their cords can interact in ways that range from uneventful to life-threatening. The distinction between separate and shared placentas is the single most important factor in understanding how umbilical cords work differently in a twin pregnancy.
What an Umbilical Cord Actually Does
A standard umbilical cord contains three blood vessels: one vein that carries oxygen-rich blood from the placenta to the baby, and two arteries that carry oxygen-depleted blood back to the placenta. These vessels are surrounded by a spongy, gel-like tissue called Wharton’s jelly, which cushions them against compression and kinking. That jelly does more than just padding. It helps maintain blood pressure inside the cord by transmitting pressure from the pulsing arteries to the vein, creating a gradient that pushes blood back toward the baby. The peripheral layer of Wharton’s jelly also lets the arteries act as pressure buffers during heartbeat pauses, smoothing out the flow so the baby gets a steady supply of oxygen and nutrients.1PubMed. The biomechanics of the umbilical cord Wharton Jelly: Roles in hemodynamic proficiency and resistance to compression
When Wharton’s jelly is reduced or missing in segments, the vessels become vulnerable to compression, which can restrict blood flow. This is a recognized cause of stillbirth, growth restriction, and fetal distress during labor. On the other end, excess Wharton’s jelly or swelling of the tissue can also interfere with blood flow and is linked to conditions like maternal diabetes.2PubMed Central. The Pathophysiology of Wharton’s Jelly and Its Impact on Fetal and Neonatal Outcomes: A Comprehensive Literature Review In twins, these problems can affect one cord and not the other, creating an asymmetry that often shows up as a size difference between the babies.
Separate Placentas Versus a Shared One
Roughly a third of twin pregnancies are monochorionic, meaning the twins share a single placenta. This happens exclusively with identical twins, when the fertilized egg splits early enough that both embryos implant into the same placental tissue. Each twin still grows its own umbilical cord, and each cord inserts into the shared placenta at its own spot, but underneath the surface their blood vessel territories overlap. Fraternal twins always develop two separate placentas (dichorionic), and so do some identical twins if the egg splits very early. When the placentas are separate, the cords work completely independently, and most of the twin-specific cord complications simply do not apply.
Even when two separate placentas sit next to each other in the uterus and appear fused on ultrasound, they do not develop blood vessel connections in humans. This is worth knowing because in some other mammals, fused placentas do form shared vessels. In human twins, blood vessel sharing only happens in the monochorionic situation, where there was never a boundary between the two territories to begin with.3PubMed. The biology of the twinning process: how placentation influences outcome
The Shared Blood Vessels That Connect Monochorionic Twins
In a shared placenta, the blood vessel networks of the two twins are linked by connections called anastomoses. These come in three types. Artery-to-artery and vein-to-vein connections sit on the surface of the placenta and allow blood to flow in both directions, balancing pressure between the twins. Artery-to-vein connections run deeper, passing through a shared chunk of placental tissue, and blood flows through them in only one direction.4PubMed Central. Accurate and simple evaluation of vascular anastomoses in monochorionic placenta using colored dye Most healthy monochorionic placentas have around eight of these connections, and the presence of surface artery-to-artery connections in particular seems to be protective. In uncomplicated shared placentas, artery-to-artery connections show up about 96% of the time.5PubMed. Prevalence, size, number and localization of vascular anastomoses in monochorionic placentas
When these connections are roughly balanced, both twins get adequate blood flow and things proceed normally. Problems arise when the flow becomes lopsided, because the deep artery-to-vein connections only go one way. If one twin consistently donates more blood than it receives, the system tips out of balance, and the consequences can be severe.
Twin-to-Twin Transfusion Syndrome
Twin-to-twin transfusion syndrome, or TTTS, is the best-known complication of shared circulation. It develops when the one-directional deep anastomoses transfer too much blood from one twin (the donor) to the other (the recipient), and the surface connections cannot compensate. The donor twin ends up with too little blood volume and produces less amniotic fluid, while the recipient twin becomes overloaded and produces excess fluid. In studies measuring umbilical vein blood flow, recipients had roughly twice the flow of donors.6PubMed. Intertwin discordance in umbilical venous volume flow: a reflection of blood volume imbalance in twin-to-twin transfusion syndrome
TTTS placentas tend to have fewer of those protective artery-to-artery surface connections. While healthy monochorionic placentas show artery-to-artery connections 96% of the time, TTTS placentas have them only about 47% of the time.5PubMed. Prevalence, size, number and localization of vascular anastomoses in monochorionic placentas Without those bidirectional safety valves, blood drifts steadily in one direction through the deep connections, and the imbalance builds.
The treatment that has emerged as the standard is fetoscopic laser surgery. A thin scope is inserted through the mother’s abdomen, and the surgeon uses a laser to seal off the shared anastomoses on the placental surface. The goal is essentially to split the shared placenta into two functionally separate territories, stopping the transfusion.7PubMed Central. Laser for twin-to-twin transfusion syndrome: a guide for endoscopic surgeons The concept of a “third circulation” between twins in a shared placenta was first identified back in 1875, but effective intervention did not arrive until fetoscopic laser techniques were developed over a century later.8PubMed. The History of Treatment of Twin-to-Twin Transfusion Syndrome
Twin Anemia Polycythemia Sequence
A subtler version of unbalanced blood sharing is twin anemia polycythemia sequence, or TAPS. Unlike TTTS, which involves large fluid shifts and dramatic differences in amniotic fluid, TAPS develops through a slow trickle of red blood cells from donor to recipient through just a handful of tiny anastomoses. The donor gradually becomes anemic while the recipient develops an excess of red blood cells (polycythemia), but neither twin shows the striking fluid imbalances seen in TTTS.9PubMed. Twin Anemia Polycythemia Sequence: Current Views on Pathogenesis, Diagnostic Criteria, Perinatal Management, and Outcome
TAPS placentas typically have very few anastomoses, with a median of around three, and artery-to-artery surface connections are present in only about 19% of cases. The donor’s side of the placenta often shows delayed tissue maturation, while the recipient’s side shows accelerated maturation and increased blood vessel density.10PubMed. Placental histology and vascular architecture in spontaneous twin anemia polycythemia sequence Because the condition progresses slowly and without obvious fluid differences on ultrasound, it can be harder to catch than TTTS, and monitoring typically relies on Doppler measurements of blood flow in each twin’s middle cerebral artery, which reflects how anemic or polycythemic the baby is.
TRAP Sequence and the Acardiac Twin
The most extreme example of shared-cord physiology is twin reversed arterial perfusion sequence, or TRAP. In this rare condition, one twin fails to develop a functioning heart and survives entirely by receiving blood pumped backward through shared artery-to-artery and vein-to-vein connections by its healthy co-twin. The blood that reaches the acardiac twin is already oxygen-poor, so its tissues develop abnormally, often resulting in a mass that barely resembles a fetus.11PubMed Central. Twin Reversed Arterial Perfusion Sequence; Characteristic Gray-Scale and Doppler Ultrasonography Findings
TRAP occurs in roughly 1 in 35,000 pregnancies. The risk to the healthy “pump” twin is significant because its heart has to work for two, and high-output heart failure can develop. The acardiac twin is not viable, so the focus of treatment is protecting the pump twin, often by occluding the blood supply to the acardiac twin.12PubMed Central. Twin Reversed Arterial Perfusion Syndrome (TRAP or Acardiac Twin)-A Case Report When cord occlusion is needed, laser techniques have shown better outcomes for the surviving twin than radiofrequency ablation. In one comparative study, radiofrequency ablation was linked to co-twin death after surgery about a third of the time, compared with under 3% for laser.13PubMed. Comparison of umbilical cord occlusion methods: Radiofrequency ablation versus laser photocoagulation
Where the Cord Plugs Into the Placenta
Even in twins whose blood supplies are completely separate, the spot where each cord inserts into the placenta matters. Normally, a cord attaches near the center of its placental territory. In a velamentous insertion, the cord’s vessels fan out across the fetal membranes before reaching the placenta, leaving them unprotected by Wharton’s jelly. In a marginal insertion, the cord attaches at the very edge of the placenta. Both of these abnormal insertions are far more common in twins than in singletons. A large population-based study of over 600,000 pregnancies found velamentous insertions in about 6% of twins and marginal insertions in about 11%, with twins being roughly four times more likely than singletons to have a velamentous cord.14PubMed Central. Prevalence, Risk Factors and Outcomes of Velamentous and Marginal Cord Insertions: A Population-Based Study of 634,741 Pregnancies
In monochorionic twins, velamentous insertion carries particular weight. It is strongly associated with selective growth restriction, meaning one twin grows significantly less than the other. One study found that discordant cord insertions, where one twin has a velamentous insertion and the other a more central one, were present in about 46% of monochorionic placentas with selective growth restriction, compared with 20% of uncomplicated ones.15PubMed. Proximate cord insertion in monochorionic twins with selective fetal growth restriction Velamentous insertion has also been identified as an independent predictor of TTTS and adverse outcomes in monochorionic pregnancies.16PubMed. Does site of cord insertion increase risk of adverse outcome, twin-to-twin transfusion syndrome and discordant growth in monochorionic twin pregnancy? In dichorionic twins, the risks from abnormal insertion are lower. One study of dichorionic twins found no significant link between abnormal cord insertion and growth discordance, intrauterine death, or composite adverse neonatal outcomes.17Ultrasound in Obstetrics & Gynecology. Significance of placental cord insertion site in twin pregnancy
However, velamentous insertion in dichorionic twins is not entirely benign either. A retrospective study found it was associated with a roughly threefold increase in the risk of preterm delivery between 32 and 34 weeks in both monochorionic and dichorionic pregnancies, and with a higher incidence of placental accreta in dichorionic pregnancies.18PubMed Central. Associations between velamentous or marginal cord insertion and risk of adverse perinatal outcomes in twin pregnancies: a retrospective cohort study
When One Cord Has Only Two Vessels
A normal umbilical cord has two arteries and one vein. Sometimes one artery fails to develop, leaving a cord with just two vessels, a condition called single umbilical artery (SUA). This is the most common congenital cord abnormality and shows up in roughly 4 to 11% of twin pregnancies, regardless of whether they are monochorionic or dichorionic.19PubMed Central. Isolated single umbilical artery in twin pregnancies and its adverse pregnancy outcomes – a case report and review of literature When it occurs, it almost always affects only one twin in the pair.
The twin with a two-vessel cord tends to be the smaller one. In one study, the baby with SUA was the smaller twin in 75% of discordant pairs, and those pairs were more than twice as likely to have a weight gap exceeding 20% compared with pairs where both cords had three vessels.20American Journal of Obstetrics & Gynecology. How Does the Umbilical Cord Work With Twins? A larger study found that twins with SUA had roughly double the odds of being small for gestational age and about triple the odds of very early preterm delivery before 28 weeks.21PubMed Central. The incidence of isolated single umbilical artery in twins and adverse pregnancy outcomes SUA is typically flagged on routine ultrasound, and when it is found, the pregnancy is monitored more closely for growth problems.
Cord Entanglement in Monoamniotic Twins
A small subset of monochorionic twins, around 1% of identical twin pregnancies, share not only a placenta but also a single amniotic sac. These monoamniotic twins have no membrane separating them, and their cords float freely in the same fluid. Cord entanglement is virtually inevitable in this situation and has been detected on ultrasound as early as eight weeks of gestation.22PubMed Central. Ultrasound diagnosis of first trimester umbilical cord entanglement in monochorionic monoamniotic twins – case report and review of the literature
The fear with tangled cords is that tightening knots could cut off blood flow to one or both babies. In practice, the outcomes are better than that fear suggests, though still serious. A systematic review of 114 monoamniotic twin pairs with documented cord entanglement found an overall survival rate of about 89%. Of the deaths that occurred, prematurity was the most common cause, and only two of 26 deaths were directly attributed to cord entanglement itself.23PubMed. Impact of cord entanglement on perinatal outcome of monoamniotic twins: a systematic review of the literature Interestingly, that same review found no significant difference in mortality between twins with documented cord entanglement and control groups, and morbidity was actually higher in the controls. Seeing cord entanglement on an ultrasound does not, by itself, predict a worse outcome. Monoamniotic twins are typically delivered early by planned cesarean, usually around 32 to 34 weeks, to reduce the ongoing risk of cord accidents.
Monitoring Twin Cords During Pregnancy
Because so many twin-specific complications relate to how the cords and placenta interact, monitoring is more intensive than in singleton pregnancies. Ultrasound is used to check each twin’s growth trajectory, amniotic fluid levels, and the Doppler patterns in the umbilical arteries. In monochorionic pregnancies with selective growth restriction, the Doppler pattern in the smaller twin’s umbilical artery is classified into types based on whether end-diastolic flow is persistently positive, persistently absent or reversed, or intermittently absent. These patterns guide decisions about timing of delivery or intervention.24PubMed. Selective intrauterine growth restriction in monochorionic twins: changing patterns in umbilical artery Doppler flow and outcomes
For dichorionic twins, the monitoring approach is simpler. Growth scans are done less frequently, and complications related to shared circulation are not a concern. One large randomized trial found that adding Doppler ultrasound of the umbilical arteries to standard biometric scans did not significantly improve outcomes compared with biometric monitoring alone, though both groups had lower-than-expected fetal death rates because of the close surveillance.25PubMed. The Doppler assessment in multiple pregnancy randomised controlled trial of ultrasound biometry versus umbilical artery Doppler ultrasound and biometry in twin pregnancy The takeaway is that regular monitoring itself is what helps, regardless of the specific technology used.
Clamping the Cord at Delivery
How and when the cord is clamped after birth has received growing attention in twin deliveries. Delayed cord clamping, waiting about 30 to 60 seconds before cutting, is now standard practice in many singleton deliveries because it allows more blood to transfer from the placenta to the baby. In twins, the logistics are trickier. Once the first twin is born, the placenta is still supporting the second twin, and there are questions about whether leaving the first cord unclamped could affect the second baby’s blood supply, especially in monochorionic twins where the placentas share vessels.
In practice, most Canadian tertiary centers report clamping the first twin’s cord after about 46 to 60 seconds. During vaginal delivery, the first baby is typically placed on the mother’s abdomen while the second is being delivered.26Journal of Obstetrics and Gynaecology Canada. Current Practices of Deferred Cord Clamping in Twin Pregnancies in Canadian Tertiary Centres A study of delayed versus early cord clamping during cesarean delivery in twins found that delaying did not increase the risk of maternal bleeding or severe neonatal complications. It did raise hemoglobin levels in the newborns and slightly increased jaundice requiring monitoring, but it also reduced the risk of neonatal anemia and intraventricular hemorrhage.27PubMed Central. Effect of delayed cord clamping on maternal and neonatal outcomes during cesarean delivery in twin pregnancies
How Twin Cord Anatomy Differs From Other Mammals
The human umbilical cord’s basic design, three vessels in a helical arrangement surrounded by Wharton’s jelly, turns out to be unusual when you look across the animal kingdom. A comparative study spanning 130 mammalian species found that the ancestral umbilical cord was unspiralled, had three vessels, and included an additional tube called the allantoic duct. In many larger mammals, cords have four vessels rather than three, and the persistence of the allantoic duct tracks with heavier birth weight. Two-vessel cords, meanwhile, are found naturally in small rodent species.28bioRxiv. Umbilical cord structure shapes feto-maternal heat exchange across mammals
One finding relevant to twins is that in humans, fused dichorionic placentas never develop blood vessel connections between the two territories. In other species like marmosets and cattle, fused placentas routinely do. In cattle, this leads to the freemartin phenomenon, where a female calf twin exposed to her male twin’s hormones through shared blood vessels develops abnormal reproductive organs. This does not happen in human fraternal twins because their placentas remain vascularly separate even when physically touching.3PubMed. The biology of the twinning process: how placentation influences outcome If one of those human twins dies in utero, there is no risk of the surviving twin losing blood into the dead twin’s circulation, because there was never a vascular bridge between them. That risk exists only in the monochorionic situation, where large shared anastomoses can allow rapid blood shifts if one twin’s heart suddenly stops.
Twin Cord Blood Banking
Cord blood collected after delivery contains stem cells that can be used for transplantation. Twin pregnancies are an interesting case for cord blood banking because they produce two collections from what may be genetically identical donors. A study examining cord blood from twin pregnancies found that processing yielded high-quality samples with average total volumes above 100 mL and cell viability exceeding 95%.29PubMed Central. ABO blood type concordance‐dependent variations in umbilical cord blood: Insights from twin pregnancies For identical twins, the two collections are immunologically interchangeable, which provides a natural backup supply. For fraternal twins, the two samples are genetically distinct, like those of any two siblings, but having both banked broadens the range of potential matches for the family. The practical challenge is that delayed cord clamping, which benefits the babies, reduces the volume of blood available for collection, so families need to weigh those priorities.