Wendy F. Hansen, M.D.
University of Iowa Hospitals and Clinics
First Published: Spring 2003
Last Revised: July 2003
Peer Review Status: Internally Peer Reviewed
Naturally, twins occur in 1 in 80 births. The era of assisted reproductive technology and the trend toward delayed childbirth have dramatically increased multiple births so that the incidence of twins is now 1 in 40 births. Dizygotic (fraternal) twinning occurs when two ova are fertilized by two sperm. They are essentially siblings residing in utero, each with its own independent chorion, amnion, and vascular network. Dizygotic twins increase with parity, with maternal age to 35, and have incidences that vary widely between populations, from 3 per 1,000 in Asians to 8 per 1,000 in Caucasians to 16 per 1,000 in Africans. In contrast, monozygotic (identical) twinning originates when a single embryo divides into two, then subsequently develops into two individuals. The incidence ranges from I in 200 to I in 300 of all births accounting for 30% of all twins and is relatively constant throughout the world. Identical siblings often share a complex anastomosis of placental vessels and chorion (monochorionic-diamniotic), less commonly an amnion (monochorionic-monoamniotic), and rarely fetal tissue and organ systems (conjoined).
Ultrasound has proved to be a powerful tool in our understanding of twinning. We now know that far more pregnancies are conceived as twins than are observed at delivery. This concept of twin conceptions outnumbering twin births was first introduced in the mid-1940s and was derived from the histopathologic examination of placentas and the finding of fetus papyraceous. The concept of a "vanishing twin" was introduced in the mid-1980s and was derived from the observation on first-trimester ultrasound of two gestational sacs with or without a fetus and the subsequent loss of one. This vanishing twin most often occurs between 5 and 9 weeks' gestation and is commonly seen with first-trimester bleeding. In the largest published series to date of over 1,000 ultrasounds in the first trimester, the inci- dence of twinning ranged from 3.2% to 5.39%. Of these, 21.2% demonstrated the vanishing twin. The prognosis for the survival of the remaining fetus is overwhelm- ingly good.
The Case for Knowing Chorionicity
Zygosity refers to the type of conception, whereas chorionicity refers to the type of placentation. We now know that all dizygotic twins have dichorionic placentations. However, 20-30% of all monozygotic twins will also have dichorionic placentas and are indistinguishable on ultrasound from dizygotic twins. In monozygotic twins, dichorionic placentation represents division of the embryo by three days of conception before the chorion and amnion have differentiated. The vast majority of monozygotic twins (70-80%) will have monochorionic placentas and represent division of the embryo between day 3-7 after the chorion has differentiated. This shared chorion contains a complex network of anastomotic vessels with numerous interconnections between the twins. Less than 1% of monozygotic twins will have monochorionic-monoamniotic placentas where there is no intervening membrane and the fetuses and cords can freely intermingle and entwine. Monoamniotic placentation represents division between days 7 and 11 after the amnion has differentiated. Conjoined twins are an even rarer subtype of monozygotic twins and represent very delayed splitting of the embryo after the primitive streak has differentiated.
Modern-day ultrasound has come a long way in determining placentation. Accuracy now approximates 100% when the ultrasound is done transvaginally between 9 and 14 weeks by experienced sonologists. When done later in the second trimester, its sensitivity and specificity approaches 90%. Establishing chorionicity early in pregnancy is enormously important and should be one of the first steps in managing any multiple pregnancy. Establishing chorionicity is important for early counseling and for risk stratification. Chorionicity will often determine techniques used in prenatal diagnosis and can be very important in counseling in special circumstances, e.g., the death of a co-twin. It is also important to many parents and to the twins themselves.
In the past, there was much emphasis given to the concept of weight "discordancy." Percents ranging from 15 to 45% were considered cause for concern, with most clinical problems occurring when the discordance reached 25%. During that time, it was always unclear whether the discordancy was twin-twin transfusion, chromosomal, or placental in origin.
Discordancy in and of itself has taken on less importance and is now stratified according to chorionicity. It is now clear that monochorionic twins have an increased risk of congenital anomalies (16% for both major and minor), along with a significant increase in perinatal morbidity and mortality compared with dichorionic twins. In monochorionic/diamniotic twins, discordancy often represents twin-twin transfusion, whether it be mild or severe, whereas in dichorionic placentations twin-twin transfusion is virtually eliminated. Cord and placenta abnormalities are far more common in both kinds of twin placentations compared to singletons and often contribute to discordance, e.g., two-vessel cords and velamentous cord insertions are nine times more common compared to singletons. In dichorionic twins, instead of using an absolute discordance percent, our concern heightens when one fetus falls below the 10th percentile, much like a singleton pregnancy.
Ultrasound is also the primary means of following the growth of twins over time as fundal heights offer little information about growth. Fetal growth in the setting of a twin pregnancy mimics that of a singleton until 28-33 weeks, when twin growth begins to level off. The exact reason for this leveling off remains unknown, although it is postulated that the limiting effect may be placental mass vs. uterine volume, etc. This leveling off increases the risk of intrauterine growth restriction (IUGR) as the pregnancy progresses, and with IUGR comes a higher risk of perinatal mortality.
Preterm Birth Risk
At the first visit when both the physician and the patient know of the twins (after congratulations and condolences are exchanged), a counseling session concerning the special risks that twins bring is essential.
While twins occur in 2.5% of all births, they account for a disproportionate share (12%) of preterm births and 15% of neonatal mortality. Traditional risk factors for preterm birth cited for singletons are not significantly associated with twins. All women with twin pregnancies are at risk. The biggest risk to women with twins is preterm birth. The average gestational age of twins at birth is 36-37 weeks, which means that 50% deliver before this time. This increased risk translates clinically into close surveillance, with many advocating every two-week visits with cervical exams from 16 weeks onward. Considerable attention has been given to the role of transvaginal ultrasound and cervical length in the antenatal surveillance of twins. It is now very clear that a short cervix is a major risk factor for preterm birth. What we do not know is whether the short cervix is the result of cervical incompetence, which we know is increased in the setting of twins, or whether this is an early preterm labor effect. To date, five small trials, two randomized and three observational, have studied the effect of cerclage placement (compared to non-cerclage group) in the setting of a shortened cervix found on ultrasound (singletons and a few twins). Studies all differ in entry criteria, in their definition of a short cervix, and in outcome analysis. The two randomized trials, both using 2.5 cm as the definition of a shortened cervix, found opposite results! The three observational studies have also differed with two showing some benefit and one retrospective study showing an increase in preterm premature rupture of membranes in cerclage patients.
At UIHC we perform cervical lengths by transvaginal ultrasound on twins beginning at 16 weeks and then every two weeks through 24 weeks. Once shortening is observed, we will often measure one week later. When the cervix shortens to 2 cm, in the absence of rupture of membranes, chorioamnionitis and regular contractions, the patient is counseled and strong consideration is given to cerclage placement.
If maternal blood volume in a singleton pregnancy increases by 40%, that of a woman with twins increases probably at least 50%. This places her at an increased risk for anemia, due to both iron and folate deficiencies, so women with twins should take a single iron tablet, a prenatal vitamin, and 1 mg of folic acid. Weight recommendations in a twin pregnancy are very dependent on the maternal prepregnancy weight (much as for a singleton). The general recommendations in normal-weight women include a targeted weight gain between 35 and 45 lbs. Women with twins are also at an increased risk for hyperemesis gravidarum, gestational hypertension, intrauterine growth restriction, and preeclampsia. Close surveillance will allow early detection and management. There is currently no prevention or in utero intervention to change these complications. Antenatal surveillance using NSTs usually starts between 32-34 weeks in normally grown twins and earlier if growth, fluid, or Doppler studies are abnormal.
The time of placing all women with twins at bedrest has passed. We now allow women with twins to continue their full activity unless cervical change is detected or preterm labor ensues. This has allowed many women with twins to remain employed and care for their families.
Twins pose a special challenge regarding genetic counseling and prenatal diagnosis. In dichorionic twins, the risk of a chromosomal abnormality of either twin at a maternal age of 33 is equal to the same risk of a singleton at a maternal age of 35. In monochorionic twins, the chromosomal status of one twin reflects the other 99% for detection of both chromosomal and structural abnormalities.
In the past decade, the association of thickened nuchal translucency between 11-14 weeks' gestation with chromosomal abnormalities has been well established. Today, the nuchal translucency measurement coupled with maternal age and crown-rump length can establish a risk for each fetus for the chromosomal trisomies 21, 18, and 13. The detection rate compares favorably to serum screening (60% serum screening vs. 75% nuchal translucency), as does the number of overall positive tests (5% serum screening vs. 5% nuchal translucency). Nuchal translucency can be used in women less than 35 with twins and can be a reasonable alternative to amniocentesis in women who want to avoid any risk to the pregnancy.
However, unlike maternal serum screening, a nuchal translucency program requires considerable training and expertise by sonographers and physicians and a commitment not only to initiate such testing, but to continue with ongoing data collection and quality assurance measures. The University of Iowa offers nuchal translucency with simultaneous consultation. It is important to recognize that nuchal translucency is not considered standard of care by ACOG; however, it is offered in many university centers.
Parents are sometimes faced with a very difficult situation when one fetus has either a chromosomal abnormality or a significant structural defect while the other twin is normal. Selective reduction can be done in special circumstances in dichorionic pregnancies. Selective reduction is contraindicated in monochorionic twins because of the high risk of fetal death and/or cerebral impairment if the other twin survives. When done between 10 and 12 weeks, the overall risk of pregnancy loss is near 5%. The more common scenario is the identification of an abnormality after 15 weeks. Selective reduction at this point in gestation often incurs a 15% risk of pregnancy loss.
Death of a Co-Twin
Intrauterine death of one fetus with continuation of the pregnancy is a rare complication occurring in 2.5% of twin pregnancies. The risk to the surviving twin is stratified according to chorionicity. With monochorionic placentations, the surviving fetus is at increased risk of perinatal mortality or long-term morbidity, primarily cerebral impairment. It is now established that the risk is at least 20% and may be higher, and is thought to result from acute hemodynamic changes as a result of placental anastomosis that manifest as an acute twin-twin transfusion. There is little evidence to justify interruption of the pregnancy after death of one twin to prevent neurologic damage to the survivor, because by the time death is diagnosed, the damage is likely to have occurred.
The death of a co-twin in a dichorionic placentation does not incur this same high risk of cerebral impairment. Management is usually expectant and dependent on gestational age.
An Intriguing Hypothesis
A group of investigators in the United Kingdom hypothesize that the "vanishing twin" in a case of monochorionic placentation is an etiology of prepartum spastic cerebral palsy. Their case is based on the high incidence of "vanishing twin," the stable incidence of cerebral palsy despite improvements in modern obstetrics, and the high risk of cerebral palsy in the surviving twin when fetal death occurs in the last half of pregnancy. They offer a mathematical model and four case reports.
In monochorionic twin pregnancies, there are placental vascular communications between the two fetuses. In 15% of these pregnancies, there is an imbalance in net blood flow between the twins, resulting in twin-twin transfusion syndrome (TTTS). In its most severe form, the recipient twin will have severe hydramnios, an increased hematocrit, and congestive heart failure, whereas the donor twin may have severe oligohydramnios and anemia and appear "stuck," with the amnion hugging the fetus in a corner of the uterus. TTTS can be mild to severe and manifests along a continuum of severity and gestational age. When TTTS is identified in the second trimester, there is a high risk of perinatal death and cerebral palsy in survivors. When untreated, there is virtually 100% mortality. Improved survival is reported with serial amnio-reductions, but approaches only 60-70% in small series. Preliminary experience with endoscopic laser coagulation of the communicating placental vessels in the second trimester is promising and under study.
These special circumstances dictate very informed consent and a team approach to offer support and guidance through complex and difficult decision making.
At the time of birth, 42% of twins will be vertex-vertex, another 38% will be vertex-nonvertex, and in about 20% of pregnancies the presenting twin will be breech. There is little argument that in the nonvertex presenting twin, a cesarean delivery is indicated. Alternatively, most physicians and patients will embark on a vaginal delivery when the presenting twins are vertex-vertex. However, there is much controversy in managing vertex-nonvertex twins. Breech extractions are still done by trained physicians throughout the country. The mandate of no breech deliveries in a singleton pregnancy does not extend to the second twin breech vaginal extraction. Many advocate a version of the second twin to vertex and if unsuccessful proceed to cesarean delivery. It is reported that about I in 20 (5%) second twins are born by cesarean when the first is delivered vaginally. Ultimately, the mode of delivery rests with the managing physician.
Twins are a challenge for both the physician and parents alike. Modern-day management of twins should include:
Twin Jeopardy - Did you know?
This information is written primarily for providers.
See related Provider Textbooks about Obstetrics and Gynecology.
See related Provider Topics Child and Teen Health, Food, Nutrition and Metabolism, High Risk Pregnancy, Obstetrics, Obstetrics and Gynecology, Pregnancy, Pregnancy and Reproduction, Twins, Triplets, Multiple Births or Women's Health.
See related Patient Textbooks about Obstetrics and Gynecology.
See related Patient Topics Child and Teen Health, Food, Nutrition and Metabolism, High Risk Pregnancy, Obstetrics, Obstetrics and Gynecology, Pregnancy, Pregnancy and Reproduction or Women's Health.
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