We now know enough about the development and potential complications associated with monochorionic twin pregnancies that the term “twin pregnancy” is no longer precise enough to be used as a medical term. We must distinguish between monochorionic and dichorionic twins.

Monochorionic twin pregnancies have unique features that substantially increase the risk of fetal death, growth restriction, and other complications. The twins share a single placenta, and their circulations are essentially linked to each other through their placental anastomoses. These linked circulations allow blood to be redirected – sometimes very rapidly – toward one twin or the other. This is not typically the case in dichorionic pregnancies.

Thus, we must always take both fetuses in a monochorionic twin pregnancy into consideration, because when one fetus is in jeopardy, the other typically is as well. This interdependency is fundamentally different from the less-entwined relationship of dichorionic twins, and makes monitoring more complicated and all the more important.

We must make the distinction between monochorionic and dichorionic twins early on – optimally, in the first trimester. With the opportunity to make this critical distinction – as well as improvements in fetal therapy and advances in ultrasound assessment that allow us to detect potential problems early – we can lay the foundation for the effective, proactive management of these at-risk pregnancies from the first trimester on.

Once the diagnosis of chorionicity is made, medical reports should specify the type of twin pregnancy that is present, rather than using what should now be considered the layman’s term “twin pregnancy.”

The potential risks of monochorionic pregnancies stem from:

– Unequal placenta sharing. In an ideal world, the twins’ single placenta is equally shared. However, it is often the case that one twin will have just 30% to 40% of the monochorionic placenta, while the other fetus has the much larger portion. Such unequal placenta sharing leads to an unequal sharing of nutrients, which can lead to growth restriction and severe low birth weight in one of the fetuses.

This type of growth restriction – known as selective intrauterine growth restriction (IUGR) – affects about 10% of all identical twins. It happens quite early in pregnancy and, as we know from singleton growth-restricted fetuses, can lead to a host of troubling complications.

That is why the fetuses in a monochorionic pregnancy can never be treated in isolation. With the early onset of growth restriction in a monochorionic pregnancy, for example, the twin with this complication faces a higher risk of in utero death – an outcome that always negatively impacts the other fetus as well.

In a dichorionic pregnancy, if a co-twin weighs 320 g at 26 weeks and is at high risk of in utero death, we typically would advise the parents to delay delivery. The extremely high likelihood of fetal death of the growth-restricted twin would not justify exposing the otherwise normally grown healthy twin to the risks of prematurity. Accepting the fetal death of the growth-restricted twin and allowing pregnancy to continue gives the larger fetus a very good chance of being healthy at birth rather than being born premature with a significant risk of prematurity-related complications.

However, in a monochorionic pregnancy, intrauterine demise of the smaller fetus could put the healthy co-twin at a significant risk for acute severe hemorrhage into the placenta and circulation of the growth-restricted twin. This carries the risk of brain, renal, and cardiac damage – or even death – of the co-twin. The option of delaying delivery beyond the point of demise of the smaller twin, therefore, is unacceptable in this setting.

Rather, the fetuses would need intensive monitoring by experts who are alert to all the potential signs of fetal deterioration. Additional options, including fetal therapy, might require even more subspecialty evaluation.

– Unequal blood volume. Blood volume also may be unequally shared. In uncomplicated pregnancies, blood is exchanged equally through the vascular anastomoses that characterize all monochorionic pregnancies. Sometimes, however, the exchange is unbalanced and blood is shunted in one direction without adequate return.

Anastomoses that are between artery and vein act as one-way valves and can lead to significant differences in volume. Artery-to-artery and vein-to-vein connections allow direct exchange in either direction, with the direction of blood flow determined by the difference in blood pressure on either side.

If one fetus develops an unstable circulation or dies, the instability or resultant drop in blood pressure causes the healthy or surviving twin to lose a large amount of blood volume across the connecting vessels and into the sick or dying twin.

This is why, when one fetus dies, the risk of death for the co-twin can be as high as 60%. It also explains why a surviving co-twin has a significant risk of brain injury.

The intertwin anastomoses account for a range of other pregnancy complications. When placenta sharing is equal but there is a significant mismatch in blood flow and blood volume, twin-to-twin syndrome (TTTS) can develop. In this scenario, the imbalance progresses to the extent that one twin becomes a “donor” of blood volume and the other twin becomes the “recipient.”

A decline in blood volume for the donor twin leads to decreased urine output to the extent that bladder filling virtually ceases and oligohydramnios may progress to anhydramnios. The recipient twin, in the meantime, urinates excessively, leading to polyhydramnios and possibly preterm labor.

TTTS develops in about 10%-15% of monochorionic pregnancies. Overall, however – if you add the approximately 10% that are affected by selective IUGR, and an unknown percentage of pregnancies that may have a bit of both problems or are complicated in other ways to this 10%-15% – I estimate that as many as one-third of monochorionic twins have some kind of significant complication.

For TTTS, endoscopic laser ablation (or laser coagulation) of placental anastomoses has been shown to be an effective treatment and a preferable first-line approach for severe cases diagnosed before 26 weeks. These therapies, however, are available only at specialized centers – a fact that adds to the value of early diagnosis of chorionicity and prospective monitoring for complications.

We cannot attempt to alleviate complications and improve survival unless a diagnosis of monochorionicity is made early. The diagnosis of chorionicity certainly is more difficult in the second trimester. However, if a patient has not had a first-trimester scan, a diagnosis should still be attempted.

Monochorionic twin pregnancies remain largely unpredictable. At 12 weeks’ gestation, however, if we have diagnosed identical twins, there are several ultrasound parameters we can measure to begin to predict how the pregnancy will proceed and what fetal complications might develop.

Some studies have shown, for instance, that a discrepancy in nuchal translucency between the co-twins of more than 60% means that there is a 60%-70% chance that TTTS will develop. There also may be some discrepancies in size of other structures that are apparent in the first trimester, such as differences in abdominal circumference, for example, as well as differences in amniotic fluid volume, or bladder size that might be helpful in planning fetal surveillance.

After initial evaluation, we generally recommend that monochorionic twins be evaluated again at 16 weeks, based on research by Dr. Liesbeth Lewi of the University Hospitals in Leuven, Belgium, showing that a combined risk assessment in the first trimester and at 16 weeks can predict selective IUGR or TTTS with greater than 80% accuracy.

In a study of 200 monochorionic diamniotic twin pregnancies, Dr. Lewi found that significant predictors of TTTS, selective IUGR, or intrauterine death in the first trimester were crown-rump length and discordant amniotic fluid volume. At 16 weeks, significant predictors were the differences between the co-twins in abdominal circumference, amniotic fluid volume, and the site of cord insertions. [The site of cord insertion was classified as velamentous, eccentric (more than 2 cm from the placental edge), or marginal (less than 2 cm from the placental edge), and a discordant cord insertion was considered to be the combination of a velamentous cord insertion in one fetus and an eccentric cord insertion in the other fetus.]

The differences between the co-twins in the ultrasound parameters were additive when measured in the first trimester and at 16 weeks. Combined risk assessment detected 58% of the fetal complications by classifying 21% of the 200 pregnancies as high risk, with a false-positive rate of 8%, while the predictive value of one assessment alone was significantly lower (Am. J. Obstet. Gynecol. 2008;199:493.e1-7).

Dr. Lewi’s research was among the literature considered recently by a panel of experts assembled by the North American Fetal Therapy Network. The panel has been working on a consensus statement that, when finalized, will make recommendations for early diagnosis of monochorionicity and basic combined risk assessment.

Doppler ultrasound (US) measurements of the umbilical arteries, which depict resistance in the blood vessels and resultant blood flow, also may be helpful. Just as with singleton pregnancies, Doppler US provides information in the monochorionic pregnancy about the vasculature of the placenta and the amount of placenta the fetuses have available for nutrient exchange.

In monochorionic pregnancies, however, Doppler US has the added benefit of being key to diagnosing and evaluating hemodynamically significant arterio-arterial anastomoses that induce variations in diastolic velocity not seen in singleton pregnancies.

The imbalance in blood flow exchange between the co-twins’ circulations – again, the primary contributor to the development of TTTS – also can be examined using Doppler assessments of two additional vascular beds: the middle cerebral artery (MCA) and the ductus venosus.

The MCA peak systolic velocity reflects how fast blood is flowing in the brain. Large differences in the MCA can point to TTTS. The ductus venosus, a unique fetal vessel that funnels a proportion of nutrient-rich umbilical venous return directly into the right atrium, similarly can be used to evaluate cardiac status. Doppler screening of the ductus venosus and MCA has its most useful role early in pregnancy.

Again, because most of the amniotic fluid from 16 weeks on is due to fetal urination, and because changes in urine output reflect changes in blood volume status, the assessment of bladder filling and amniotic fluid volume reveals much about blood volume status and possible TTTS.

Whenever we see a monochorionic twin pregnancy, therefore, we face a range of questions: What are the sizes of the fetuses? Is there a discrepancy? What is the ultrasound end-diastolic velocity in each twin? Is it normal? Or, is there variability in the waveform, which is indicative of hemodynamically significant arterio-arterial anastomoses? Is the amniotic fluid volume normal? What do the bladders look like? Does one fetus have a bladder that’s barely filling?

By regularly asking these questions – and using the pregnancy as its own control – we will be alert to the potential problems associated with monochorionicity and more able to proactively plan our monitoring schedules. A new discrepancy or a change from a previous exam might mean seeing the patient weekly as opposed to every 2 or 3 weeks. Frequent monitoring is prudent throughout pregnancy as severe TTTS can develop until 22-23 weeks’ gestation, even when findings are normal at 18 weeks. Moreover, milder forms of TTTS, as well as milder forms of selective IUGR, can develop even later.

Dr. Baschat is with the department of obstetrics, gynecology, and reproductive sciences at the University of Maryland, Baltimore. He is a recognized national expert in fetal therapy, including various intrauterine surgical procedures. Dr. Bashchat reported that he has no disclosures regarding this article.

Copyright (c) 2009 Elsevier Global Medical News. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

鉴于目前我们已经清楚知道单绒毛膜双胎妊娠的发生机制和潜在并发症，因此用“双胞胎”作为医学术语已不够准确。我们必须区分“双胞胎”是单绒毛膜性还是双绒毛膜性。

单绒毛膜双胎妊娠具有独特的临床特征，可显著增加死胎、胎儿生长受限及其他并发症的风险。此类双胞胎共用一个胎盘，本质上一胎儿的血液通过胎盘血管吻合网输入到另一胎儿。相互联系的血液循环使得血液在胎儿间迅速进行交换。而这种情况在双绒毛膜双胎妊娠中并不多见。

因此在单绒毛膜双胎妊娠中，我们必须始终考虑两个胎儿，因为其中一个胎儿常因另一胎儿处于危险之中而受累。这种依存关系与较少交织在一起的双绒毛膜双胞胎有着根本的不同，使得胎儿检测变得更为复杂和重要。

我们必须尽早（最好在怀孕的头三个月）区分双胞胎是单绒毛膜妊娠还是双绒毛膜妊娠。在明确这个重要问题后，借助不断进步的胎儿超声检查和治疗方法，我们就能在妊娠早期发现潜在问题，为积极有效地处理妊娠的危险状况打下基础。

在进行绒毛膜相关检查时，报告结果应指明双胎妊娠的类型，而不应使用“双胞胎妊娠”这样的非专业术语。

单绒毛膜双胎妊娠的潜在风险源于：

1. 胎盘共享不平衡。在理想状态下，双胞胎平均共享一个胎盘。然而，单绒毛膜双胎妊娠胎盘中经常是一个胎儿只占到30%～40%，而另一个胎儿则占用了大部分。这种不平衡导致胎盘养分分配不均，进而导致其中一个胎儿生长受限以及出生体重过低。

此类生长受限也称为选择性宫内生长受限（IUGR），在双胞胎的发生率约为10%。它发生在妊娠早期，像我们熟知的单胎妊娠的胎儿生长受限一样，可导致一系列棘手的并发症。

因此，我们不能孤立看待单绒毛膜双胎妊娠的胎儿。例如，在单绒毛膜双胎妊娠胎儿生长受限发生早期，并发症胎儿可能面临更高的宫内死亡风险，并对另一胎儿产生不利影响。

在双绒毛膜双胎妊娠中，如果其中一个胎儿在26周仅重320克，发生宫内死亡风险很高时，我们通常会建议推迟产妇的分娩时间。但是，并不能因双胎妊娠生长受限胎儿具有极高死胎风险，就让另一正常生长的健康胎儿提前生产。接受生长受限胎儿死亡的事实并继续妊娠，可保证个头大的胎儿出生时健康，因此，不应提前生产以免发生早产相关并发症。

但在单绒毛膜双胎妊娠中，较小胎儿发生宫内死亡可能会显著增加健康胎儿发生胎盘内急性重症出血及生长受限胎儿循环内急性重症出血的风险。这将可能损害健康胎儿的大脑、肾脏和心脏，甚至导致死亡。在这种情况下，选择在较小胎儿发生宫内死亡后推迟分娩是不可取的。

如若妇产科专家警告胎儿可能有情况恶化的征象，则需对胎儿进行密切监视，也可选择对胎儿进行治疗，但这可能需要更专业的评估。

2. 血容量不平衡。双胎妊娠的血量可能也是不均衡的。单绒毛膜双胎妊娠与普通妊娠一样，胎儿也是通过胎盘血管吻合网与母体进行血液交换。但有时候因回路血管不足，血液只能向一个方向分流，从而导致血液交换发生失衡。

动脉和静脉间的吻合支就像一个单向的阀门，可引起血量的显著性差异。而动脉间或静脉间的交通支则允许血液直接进行双向流通，血流方向由血管两端的血压差决定。

如果其中一个胎儿发生循环不稳或者死亡，大量血液就会因这种不稳定或血压下降通过交通支从双胎中健康或存活的胎儿流向患病或死亡的胎儿。

这就是当一个胎儿死亡后，双胎的另一个体死亡率会高达60%的原因。这也解释了存活双胎个体脑损伤风险为什么会显著升高。

双胞胎血管吻合还引起其他一系列的妊娠并发症。当胎儿平分胎盘，但血流和血容量发生失衡时，可发生双胎输血综合征。在这种情况下，失衡继续发展，常常使一个胎儿成为供血者，另一个胎儿成为受血者。

双胎中供血胎儿血容量下降可导致尿量排出减少，膀胱停止充盈，引起羊水过少甚至无羊水。相反，受血胎儿尿液排出增加，导致羊水过多，可能引发早产。

单绒毛膜双胎妊娠的双胎输血综合征发生率约10%～15%。如果加上胎儿宫内发育迟缓10%的发生率，以及两种情况共同存在或其它复杂因素引起的妊娠问题（发生率不明），总体说来估计多达三分之一的单绒毛膜双胞胎患有某种严重的并发症。

通过胎盘血管吻合网内镜激光烧蚀（或激光凝固）治疗双胎输血综合征，目前已证明是一种有效的治疗方法，也是孕26周前确诊的重症患者的一线治疗方案。但这些治疗只能在专科中心进行，这一现实大大提高了早期绒毛膜性诊断和并发症预防性监测的价值。

只有在妊娠早期诊断出单绒毛膜性，我们才能尝试去减轻并发症并改善胎儿存活率。在妊娠的第4～6个月，进行绒毛膜性诊断无疑更为困难。但如果患者在孕早期没有做相关检查，仍应尝试进行诊断。

目前，单绒毛膜双胎妊娠仍难以预测。但在孕12周，如果已经确诊是双胎妊娠，可通过超声检测几个参数，预测妊娠过程以及胎儿可能发生的并发症。

例如，有研究表明，采用超声检查测量双胞胎颈项皮肤透明层厚度差异达60%以上者，意味着将有60%～70%的几率罹患双胎输血综合征。此外，在妊娠前3个月，还可能有其他一些结构大小上的差异，如腹围、膀胱以及羊水量的差异，可能有助于进行胎儿监测。

经初步评估后，我们通常建议在孕16周对单绒毛膜双胞胎进行第二次检查。比利时鲁汶大学医院的Liesbeth Lewi博士在研究中发现，孕早期和孕16周的联合风险评估对双胎输血综合征或选择性胎儿宫内发育迟缓预测准确率可高达80%。

该研究共纳入200例单绒毛膜双羊膜腔双胎妊娠孕妇。Lewi博士发现，孕早期时双胎输血综合征、选择胎儿宫内发育迟缓或胎儿宫内死亡的重要预测因子为头臀长度以及羊水量不协调。在孕16周，重要预测因子则为双胞胎腹围、羊水量和脐带附着部位的差异。[脐带附着部位分为帆状、偏心性（超出胎盘边缘大于2厘米）、边缘性（超出胎盘边缘小于2厘米）以及不一致混合性附着，即一胎儿呈脐带帆状附着而另一胎儿呈偏心性附着。]

在孕早期和孕16周的超声检查中，双胞胎之间的检查参数差异性增加。联合风险评估发现归类高风险的21%孕妇中，有58%检查出胎儿并发症，其中假阳性率为8%，而单一评估的预测值明显更低（Am. J. Obstet. Gynecol. 2008;199:493.e1-7）。

Lewi博士的研究论文最近正在接受北美胎儿治疗网络专家小组的审议。该小组正设法通过协商达成最终一致性意见，对单绒毛膜性的早期诊断和基本联合风险评估提出建议。

多普勒超声可以反映脐动脉血管阻力及其血流，因此可能有助于单绒毛膜双胎妊娠并发症的风险评估。多普勒超声可提供单胎妊娠中进行营养交换的胎盘血管和数量的信息，类似地，其也可提供双胎妊娠中的此类信息。

在单绒毛膜双胎妊娠中，多普勒超声还有其他优势，主要是可根据血流动力学的特点，诊断和评价显著的动脉间吻合，这种现象会引起在单胎妊娠中少见的舒张期速度的变化。

双胎输血综合征发生的主要原因——双胎血液循环中血流交换的失衡，也可通过对大脑中动脉（MCA）和静脉导管这两个血管床行多普勒超声而检查出来。

大脑中动脉峰值收缩速度反映血液流入大脑的速度。该值相差较大提示患有双胎输血综合征。静脉导管是胎儿时期存在的一根独特的血管，营养丰富的脐静脉血通过此导管直接回流到右心房，同样我们可以用它来评价心脏情况。利用多普勒超声对孕早期胎儿的静脉导管和大脑中动脉进行筛查是十分有用的。

自孕16周起大部分的羊水来源均为胎儿尿液，且尿量变化可以反映血容量的变化状况，因此超声检查膀胱充盈和羊水量，可以反映血容量状况及发生双胎输血综合征的可能性。

因此，我们每遇到一例单绒毛膜双胎妊娠，就应该问自己一系列的问题：胎儿的大小？是否有差异？每个双胞胎的超声舒张末期速度是多少？是否正常？或反映血流动力学的波形是否有变化？是否发生显著的动脉间吻合？羊水量是否正常？膀胱充盈怎么样？是否有胎儿的膀胱基本未充盈？

通过经常问自己这些问题，并利用孕妇进行自身对照，我们就会对这些与单绒毛膜性相关的潜在问题具有敏感性，并能更积极地规划监测方案。如果在检查中发现有新的差异或改变，则意味着需要每周而不是每2～3周对病人随访。有必要在整个妊娠期对孕妇进行经常性监测，因为即便孕18周检查正常，重度双胎输血综合征仍可能在孕22～23周前发生。此外，轻度的双胎输血综合征及选择性胎儿宫内发育迟缓的发生时间甚至更晚。

Baschat博士目前供职于巴尔的摩马里兰大学生殖医学中心和妇产科，是美国著名的胎儿治疗专家，擅长各种胎儿宫内手术治疗。Bashchat博士声称没有与本文相关的利益披露。

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