Twin Gestation with Vasa Previa Case File

Twin Gestation with Vasa Previa Case File

Eugene C. Toy, MD, Patti Jayne Ross, MD, Benton Baker III, MD, John C. Jennings, MD

CASE 8

A 31-year-old G4P3003 woman at 36 weeks’ gestation is admitted to the labor and delivery unit for evaluation of uterine contractions. She has a known twin pregnancy, and throughout the pregnancy, she had significant nausea and vomiting, but otherwise her prenatal course has been unremarkable. Serial ultrasound examinations have been performed showing concordant growth of the twins. She takes prenatal vitamins, an iron supplement, and folic acid. On examination, blood pressure (BP) is 110/70 mm Hg, pulse is 80 beats per minute (bpm), and respiratory rate is 18 breaths per minute. Fundal height is 41 cm. Her cervix is 4 cm dilated and 90% effaced. Ultrasound examination reveals a twin pregnancy with a dividing membrane, and adequate amniotic fluid. The twins are presenting vertex/vertex. After 2 hours of labor, the patient dilates to 6 cm. Artificial rupture of membranes is undertaken to allow for a fetal scalp electrode of twin A. A moderate amount of vaginal bleeding is noted after rupture of membranes. Twin A’s fetal heart rate tracing initially was in the 140 bpm baseline, and then increases to 170 bpm, and now has a sinusoidal appearance.

» What is the most likely diagnosis?

» What is the cause of this condition?

» What is the next step in management?

ANSWER TO CASE 8:

Twin Gestation with Vasa Previa                                                      

Summary: A 31-year-old G4P3 woman at 36 weeks’ gestation with a twin pregnancy presents in labor. Upon rupture of membranes, there is moderate vaginal bleeding noted. Twin A has fetal tachycardia and now a sinusoidal heart rate pattern.

• Most likely diagnosis: Twin gestation with vasa previa.
• Cause of this condition: The exact pathophysiologic mechanism of vasa previa is not known, but it is associated with a velamentous cord insertion (explained below), accessory placental lobes, and second trimester placenta previa. The incidence of vasa previa is increased in pregnancies conceived by in vitro fertilization (IVF).
• Next step: Stat cesarean and alert pediatricians for likelihood of anemia in twin A.

ANALYSIS

Objectives

1. Become familiar with the mechanisms responsible for twinning.
2. Understand the implications of twin gestation for a pregnancy (both maternal and fetal effects).
3. Recognize risk factors for and complications of vasa previa.

Considerations

This 31-year-old woman presents with a known twin gestation and ultrasound findings consistent with a vasa previa, where a fetal vessel overlies the internal cervical os. This presents a danger to the fetus when rupture of membranes occurs, as the fetus can rapidly exsanguinate. Prenatal diagnosis of this condition is of the utmost importance, as there is nearly a two-fold increased chance of survival with prenatal diagnosis; unfortunately, it is difficult to diagnose prenatally. The twin gestation has its own set of possible complications that must be considered. These include the increased risk of congenital anomalies, preterm labor, preeclampsia, postpartum hemorrhage, and maternal death. Finally, the sinusoidal heart rate pattern is a rare finding (a category III ominous pattern), and usually associated with severe fetal anemia.

APPROACH TO:

Multiple Gestation                                                        

DEFINITIONS

VELAMENTOUS CORD INSERTION: Umbilical vessels separate before reaching the placenta, protected only by a thin fold of amnion, instead of by the cord or the placenta itself; these vessels are susceptible to tearing after rupture of membranes.

VASA PREVIA: Umbilical vessels that are not protected by cord or membranes, which cross the internal cervical os in front of the fetal presenting part; this most commonly occurs with a velamentous cord insertion or a placenta with one or more accessory lobes.

BILOBED OR SUCCENTURIATE-LOBED PLACENTA: A placenta with either one or more accessory lobes.

MONOZYGOTIC TWINS: Twins formed by the fertilization of one egg by one sperm.

DIZYGOTIC TWINS: Twins formed by the fertilization of two eggs by two sperm.

CHORIONICITY: The number of placentas in a twin or higher order gestation; in monozygotic twins, can either be monochorionic or dichorionic. Dizygotic twins are always dichorionic.

AMNIONICITY: The number of amniotic sacs in a twin or higher order gestation; monozygotic twins may be monoamnionic or diamniotic whereas dizygotic twins are always diamniotic.

CLINICAL APPROACH

The incidence of twin gestation has dramatically increased in the United States over the last two or three decades. This is a result of the increasing use of infertility treatments, including ovulation induction and in vitro fertilization. This dramatic increase has created a new public health concern, as twin pregnancies are associated with a higher rate of preterm delivery and all of the complications associated with it. The other complications of twin gestation include a higher rate of congenital malformations, a two-time increased risk of preeclampsia and postpartum hemorrhage, and twin– twin transfusion (TTT) syndrome.

There are two types of twinning: monozygotic and dizygotic. Monozygotic twins are formed when one egg is fertilized by one sperm followed by an error in cleavage; the incidence is not related to race, heredity, age, or parity. The exact mechanism of monozygotic twinning is not known, but may be caused by a delay in normal events, such as when tubal motility is decreased. Oral contraceptives (OCPs) slow tubal motility, so it is important to know if a mother has used OCPs within 3 months of becoming pregnant. This is associated with an increased incidence of twinning.

Table 8–1 • CHORIONICITY AND AMNIONICITY OF MONOZYGOTIC TWINS
Timing of Division (after Fertilization)	Resulting Chronicity and Amnionicity
First 72 hours	Dichorionic/diamniotic
Day 4-8	Monochorionic/diamniotic
Day 8-12	Monochorionic/monoamniotic
After day 12	Conjoined twins

The second way to categorize twins is by their membranes: the chorionicity and amnionicity of monozygotic twins is determined by the timing of division of the embryos (see Table 8– 1, and Figures 8– 1 and 8– 2). Relative to dizygotic twins, monozygotic twins are associated with a higher incidence of discordant growth and malformations, with monochorionic twins being associated with a much higher rate of spontaneous abortion.

Dizygotic twins are formed by the fertilization of two eggs by two sperms. The incidence is influenced by race, heredity, maternal age, parity, and fertility drugs. The incidence is 1:100 in white women and 1:80 in black women. The rate of dizygotic twinning increases with maternal age and peaks at 37 years. There is an increased incidence of a twin pregnancy when the mother is a dizygotic twin. Fertility treatments are responsible for many twin gestations. Clomiphene induces ovulation and promotes the maturation of multiple follicles, therefore increasing the number of eggs released during ovulation and available for fertilization. In vitro fertilization involves the transfer of two to four embryos to the uterus. If more than one implants, a twin or higher order gestation occurs. All dizygotic twins are dichorionic/ diamniotic.

Figure 8–1. Twin gestation with thick dividing membrane indicating dichorionic, diamniotic

membrane. The bottom arrow points to yolk sac and the top arrow points to dividing membrane.

(Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al. Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010, Figure 39–7a.)

Figure 8–2. Twin gestation with thin dividing membrane indicating monochorionic, diamniotic

membrane. This is a monozygotic twin. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al. Williams Obstetrics. 23rd edition. Figure 39–7b.)

In any kind of twin gestation, it is important to remember that maternal screening and physiology may be different from that in a singleton pregnancy. Increased maternal serum α-fetoprotein (msAFP) may be misleading, especially in the case of a vanishing twin where only one fetus is seen on ultrasound. Nausea and vomiting can be increased in a twin gestation, due to higher serum levels of hCG. H emodynamically, blood volume and stroke volume are increased more than in a singleton pregnancy. However, the red cell mass increases proportionately less, so there is greater physiologic anemia. Blood pressure at 20 weeks is usually lower than in a singleton pregnancy, but is higher by delivery. Finally, there is a greater increase in size and weight of the uterus, as might be expected.

Maternal complications more common with multiple gestations include preeclampsia, gestational diabetes, anemia, deep venous thrombosis, postpartum hemorrhage, and the need for cesarean delivery. Fetal or placental complications include preterm delivery, intrauterine growth retardation (IUGR), polyhydramnios, stillbirth, fetal anomalies, placenta previa, abruption, and twin– twin transfusion syndrome. In TTT syndrome, one twin is the donor and the other the recipient such that one twin is larger with more amniotic fluid and the other twin smaller with oligohydramnios. Treatment includes laser ablation of the shared anastomotic vessels at special centers, or serial amniocentesis for decompression. When there is no dividing membrane between the twins, cord entanglement can occur, leading to a 50% perinatal mortality rate. Thus, an important part of the ultrasound evaluation of twin gestations is identification of a dividing membrane.

When a multiple gestation is diagnosed, the patient should be followed in a high-risk clinic with serial ultrasound examinations for growth and comparison weight, and careful monitoring for the above complications. Delivery can be vaginal when both twins are presenting as vertex. When the first twin is nonvertex, cesarean delivery is usually performed. When the first twin is vertex, delivery of the nonvertex second twin is individualized.

Vasa previa is a serious condition that can cause fetal death rapidly after rupture of membranes. Survival is increased more than two-fold by prenatal diagnosis, from 44% to 97%. However, prenatal diagnosis is difficult. It is difficult to identify on vaginal examination, especially before membrane rupture, and ultrasound may give some hint. Currently, accepted risk factors are a bilobed, succenturiatelobed, or low-lying placenta, multifetal pregnancy, and pregnancy resulting from in vitro fertilization. Women with these risk factors or suggestive ultrasound findings should have a color Doppler ultrasound. If vasa previa is identified, a planned cesarean delivery should take place before rupture of membranes, around 35 to 36 weeks of gestation. Digital vaginal examination is contraindicated in cases of vasa previa.

Because fetal blood volume at term is only 250 to 500 cc, it is not hard to imagine that the fetus may exsanguinate within minutes of an umbilical vessel being torn. Fetal heart rate abnormalities such as tachycardia, recurrent decelerations, prolonged bradycardia, and a sinusoidal pattern can indicate serious fetal compromise and should prompt evaluation for its cause. If fetal bleeding is uncertain, the Apt test and Kleihauer–Betke test can be used to differentiate fetal from maternal blood.

COMPREHENSION QUESTIONS

8.1 A 28-year-old G1P0 woman is diagnosed as having a twin gestation at 15 weeks’ gestation. Careful examination of the membranes reveals that there is a very thin membrane between the two fetuses. Which of the following statements is most accurate?

A. It is likely that one fetus is a male and the other a female.

B. It is likely that this is a dizygotic gestation.

C. It is likely that this is a monozygotic gestation.

D. It is likely that there are two separate placentas.

8.2 A 25-year-old G2P1001 woman at 27 weeks’ gestation has been followed for twin gestation. She is undergoing her third ultrasound examination today. Her ultrasound findings are as follows:

	Twin A	Twin A
Estimated weight	500 g	1100 g
Amniotic fluid	2 cm	26 cm

Which of the following is the best next step for this patient?

A. Chorionic villus sampling

B. Repeat ultrasound in 3 weeks

C. Laser ablation of vessels

D. Revision of dates for twin B

8.3 A 32-year-old G1P0 woman undergoes an IVF pregnancy cycle and becomes pregnant with triplets. She has been followed in a high-risk obstetrics clinic with an uncomplicated pregnancy course. She arrives to the hospital labor and delivery unit at 30 weeks’ gestation with a blood pressure of 150/ 100 mm Hg, and 2+ proteinuria. Additionally, she complains of dyspnea. Her oxygen saturation is 82% on room air. She is contracting every 4 minutes. The patient is diagnosed with preeclampsia. Which of the following statements is most accurate?

A. Await spontaneous labor

B. Induce labor immediately

C. Cesarean at 34 weeks’ gestation

D. Cesarean at 39 weeks’ gestation

ANSWERS

8.1 C. The ultrasound findings are consistent with monochorionic, diamniotic twins, since there is only a thin membrane between the two gestations. Since a dizygotic gestation always gives rise to a dichorionic diamniotic gestation, this patient must have a monozygotic pregnancy which split at 4 to 8 days after fertilization. A monozygotic pregnancy is at greater risk for IUGR, stillbirth, and TTT syndrome.

8.2 C. The large discrepancy of fetal weight and amniotic fluid volume between the two gestations is consistent with TTT syndrome. The best treatment is laser ablation of the shared vessels, but this procedure is only available at select centers. Another option is serial amniotic fluid reduction. In TTT syndrome, one twin acts as the donor (smaller) and the other as the recipient (larger). A high stillbirth risk exists with this condition.

8.3 B. This patient likely has pulmonary edema due to preeclampsia as well as the increased plasma volume due to multiple gestations. The higher the number of pregnancies, the more the plasma volume, and greater the risk of pulmonary edema. This patient should be placed on intravenous furosemide to decrease intravascular volume, magnesium sulfate for seizure prophylaxis, and plans made for delivery. Although deep venous thrombosis (DVT) and pulmonary embolism is always a consideration in a pregnant woman with dyspnea and hypoxemia, pulmonary edema would be more likely. The chest radiograph would be helpful to differentiate the two conditions (infiltrates with pulmonary edema, clear in pulmonary embolism). Tocolysis and corticosteroids would be useful in isolated preterm labor, although many experts avoid their use in multiple gestations because of the risk of pulmonary edema.

    CLINICAL PEARLS    

» The two types of twin gestations are mono- and dizygotic. Monozygotic twins are associated with a higher rate of anomalies and maternal complications. » Maternal effects of pregnancy are enhanced in twin gestation—increased nausea and vomiting, greater “physiologic” anemia, greater increase in blood pressure after 20 weeks, and greater increase in size and weight of the uterus. » TTT syndrome should be suspected with a substantial discordance of the twins and discrepancy of the distribution of the amniotic fluid volume. » Twin gestation without a dividing membrane is associated with a high stillbirth rate due to cord entanglement. » Vasa previa is a serious condition that can cause rapid fetal demise after rupture of membranes. » Prenatal diagnosis of a vasa previa is best made by ultrasound with color Doppler, and management is planned cesarean delivery before rupture of membranes.

REFERENCES

Chasen, ST, Chervenak, FA. Twin pregnancy: prenatal issues; 2012, Accessed 10.02.2012. 

Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe J. Abnormalities of the placenta, umbilical cord, and membranes. In: Williams Obstetrics. 24th ed. New York, NY: McGraw-Hill; 2014: 577-587. 

Cunningham FG, Leveno KJ, Bloom SL, Spong CY, Dashe J. Multifetal gestation. In: Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2014:859-889. 

Lockwood, CJ, Russo-Stieglitz, K. Vasa previa and velamentous cord; 2014 Accessed 10.06.2014. 

Oyelese Y, Sulian JC. Placenta previa, placenta accreta, and vasa previa. ACOG Clinical Expert Series. Obstet Gynecol. 2006;107:927-941. 

Strehlow S, Uzelac P. Complications of labor & delivery. In: DeCherney AH, Nathan L, eds. Current Diagnosis & Treatment of Obstetrics & Gynecology. 11th ed. New York, McGraw-Hill; 2012.

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