T 2: CASES. 159. Case 25 A 38 - year - old woman with a twin pregnancy. Mrs
Akinte is 38 - year - old primigravida admitted with severe hyperemesis which
has ...
Mrs Akinte is 38-year-old primigravida admitted with severe hyperemesis which has improved with hydration and antiemetics. She has a pelvic ultrasound which confirms a twin pregnancy about which she is delighted as it is a pregnancy following treatment with clomifene citrate for anovulatory primary subfertility.
What are the predisposing factors for twin pregnancy? These include a family history or previous history of multiple births, increased maternal age, ovulation induction (with clomifene 10%, with gonadotrophins 30% and in vitro fertilization [IVF] 25–30%) and race (Japanese 7/1000 pregnancies, Nigerian 40/1000)
more complex depending on the timing of the division of the embryo: • Embryo splits at 3 days: two chorions, two amnions (dichorionic, diamniotic) • Embryo splits at 4–7 days: single placenta, one chorion, two amnions (monochorionic, diamniotic) • Embryo splits at 8–12 days (rare): single placenta, one chorion and one amnion (monochorionic, monoamniotic) • Embryo splits at 13 days (very rare): conjoined or Siamese twins The scan shows the twins to be dichorionic and diamniotic at 10 + 3 weeks’ gestation and the heart beat of both the fetuses are seen. Mrs Akinte wishes to know if the twins are
KEY POI NT Twins account for about 1% of all pregnancies with two-thirds being dizygotic and one-third monozygotic. The incidence of triplets is 1/4000. There is increased incidence as a consequence of assisted reproductive techniques.
Mrs Akinte wants to know the causes for twin pregnancy and also the different types of twins. What do you tell her? Twin pregnancy occurs when two or more ova are fertilized to form dizygotic (non-identical) twins, or a single fertilized egg divides to form monozygotic (identical) twins. In a dizygotic twin pregnancy, each fetus has its own placenta (either separate or fused), amnion and chorion, whereas in a monozygotic pregnancy, the situation is
Obstetrics and Gynaecology: Clinical Cases Uncovered. By M. Cruickshank and A. Shetty. Published 2009 by Blackwell Publishing. ISBN 978-1-4051-8671-1.
identical.
How can you tell if the twins are monozygotic or dizygotic? While a monochorionic placentation on scan suggests identical (monozygotic twins), with a dichorionic placenta it is not possible to determine zygosity unless the twins are of discordant gender. Determination of zygosity may be useful for: • Assisting with medical decisions for the twins in later life (e.g. to establish any genetic risk of illness) • Participation in twin research studies • Simply answering the inevitable questions from friends, relatives or strangers Zygosity can only be conclusively determined by DNA fingerprinting, which requires amniocentesis, chorionic villus sampling (CVS), cordocentesis and after delivery by DNA fingerprinting of cord blood or sending a small swab of cheek cells or a blood sample to the laboratory. Determination of chorionicity can be performed by ultrasonography and relies on the assessment of fetal gender, number of placentas and characteristics of the membrane between the two amniotic sacs. Different-sex 159
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Case 25 A 38-year-old woman with a twin pregnancy
160 Part 2: Cases
Dizygotic (non-identical)
Dichorionic
Monozygotic (identical)
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(a)
Monochorionic Figure 25.1 Dizygotic and monozygotic twins. In dichorionic twins the inter-twin membrane is composed of a central layer of chorionic tissue sandwiched between two layers of amnion, whereas in monochorionic twins there is no chorionic layer present.
twins are dizygotic and therefore dichorionic, but in about two-thirds of twin pregnancies the fetuses are of the same sex and these may be either monozygotic or dizygotic (Fig. 25.1). Similarly, if there are two separate placentas the pregnancy is dichorionic. In dichorionic twins the inter-twin membrane is composed of a central layer of chorionic tissue sandwiched between two layers of amnion, whereas in monochorionic twins there is no chorionic layer present. Dichorionic twins can be easily distinguished by the presence of a thick septum between the chorionic sacs. This septum becomes progressively thinner to form the chorionic component of the inter-twin membrane, but remains thicker and easier to identify at the base of the membrane as a triangular tissue projection, or ‘lambda’ sign. Sonographic examination of the base of the inter-twin membrane at 10–14 weeks’ gestation for the presence or absence of the lambda sign (Fig. 25.2) provides reliable distinction between dichorionic and monochorionic
(b) Figure 25.2 Ultrasound appearance of monochorionic (left) and dichorionic (right) twin pregnancies at around12 weeks’ gestation. The ‘lamda’ sign is seen in the dichorionic set of twins and the ‘T’ sign (with the very thin inter-twin membrane) in the monochorionic set of twins.
pregnancies. With advancing gestation there is regression of the chorion laeve and the ‘lambda’ sign becomes progressively more difficult to identify.
Mrs Akinte wishes to know the importance of prenatal determination of chorionicity. What information would you give her? Chorionicity, rather than zygosity, is the main factor determining pregnancy outcome. In monochorionic
Case 25 161
risk of trisomy and is an option for screening with multiple pregnancies. Invasive prenatal diagnosis is challenging as there are at least two fetuses to sample correctly and should be undertaken in a tertiary referral centre. The procedure chosen will depend on chorionicity. Both amniocentesis and CVS risk contamination – amniocentesis where double sac sampling occurs and CVS where chorions are not separately sampled. Procedure-related miscarriage rates appear to be similar to those for singleton pregnancies.
KEY POINT recipient there is usually a large bladder and polyhydramnios and the smaller anuric donor is held fixed to the placenta by the collapsed membranes of the anhydramniotic sac.
twins the rates of miscarriage, perinatal death, preterm delivery, fetal growth restriction and fetal abnormalities are much higher than in dichorionic twins. Death of a monochorionic fetus is associated with a high chance of sudden death or severe neurological impairment in the co-twin. Twin–twin transfusion syndrome (TTTS), an imbalanced flow of blood from one twin to another, occurs in 10–15% of monozygotic twins who share a placenta (Fig. 25.3). The implications of this are very serious for the survival (perinatal mortality of >80%) and health of both twins and they would require close monitoring during the pregnancy.
In view of her age, Mrs Akinte is concerned about the risk of Down’s syndrome. What screening investigations would you offer her? Fetal abnormality is more common in multiple pregnancies – both the maternal age-specific chromosomal disorders (as increasing maternal age is a risk factor for multiple birth) and fetal anatomical disorders (seen more with monochorionic than dichorionic twins). Serum screening in multiple pregnancy is not reliable, as it may identify only about 45% of affected fetuses for a 5% false positive rate. Nuchal translucency (NT) assessment (ultrasound measurement of the translucency of the nuchal fold in the fetal neck between 10 and 14 weeks) identifies about 70% of individual fetuses at high
If one fetus is detected as abnormal, selective termination (if desired) with intracardiac potassium chloride in dichorionic twins must be accurately targeted. Selective termination in monochorionic pregnancies risks co-twin sequelae, but cord occlusion can be considered.
Mrs Akinte opts to have the NT scan at 12 weeks which shows the fetuses to be at low risk for Down’s syndrome. She asks about the risks associated with multiple pregnancy.
What do you tell her? Multiple pregnancies are considered high-risk because: • Increased risk of prematurity – the mean gestation for twins is 37 weeks and for triplets 31 weeks • Higher risk of congenital abnormality associated with multiple pregnancies (×2–4 the rate in singleton pregnancies) • Higher rates of cerebral palsy found in twins (1–1.5%) and triplets (7–8%) • Perinatal mortality rate for twins is significantly higher than singletons (×5) and even higher for triplets (×6). • Smaller babies – fetuses tend to be individually smaller than those in a singleton pregnancy because of greater demand for nutrients and slower in utero growth, i.e. light-for-dates. Monozygotic twins tend to be smaller than dizygotic twins. • Death of one fetus. Death of one fetus in dichorionic pregnancies carries a risk of death or handicap of 5–10% to the remaining fetus. This is mainly because of preterm delivery, which may be the consequence of release of cytokines and prostaglandins by the resorbing dead placenta. In monochorionic twins, there is at least a 30% risk of death or neurological handicap to the co-twin because, in addition to preterm delivery, there is a risk of
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Figure 25.3 Twin–twin transfusion syndrome (TTTS). In the larger
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Box 25.1 Complications specific to monochorionic twin pregnancy
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Twin–twin transfusion syndrome (TTTS) with placental vascular anastomosis with unequal distribution of blood between the twins. Approximately 10–15% of monochorionic twin pregnancies may be affected with TTTS. The donor twin becomes anaemic, hypovolaemic, oligohydramniotic and growth restricted. The recipient becomes polycythaemic, hypovolaemic and polyuric with polyhydramniosis and hydrops. Twin reversed arterial perfusion sequence (TRAP) is found in approximately 1% of monozygotic twin pregnancies (acardiac twinning). The underlying mechanism is thought to be disruption of normal vascular perfusion and development of the recipient twin because of an umbilical arterial–arterial anastomosis with the donor or pump-twin.
Box 25.2 Management of twin pregnancies Monochorionic twins should be scanned fortnightly from 16 weeks to detect twin–twin transfusion syndrome (TTTS). The pathognomonic features of severe TTTS by ultrasonographic examination are the presence of a large bladder in the polyuric recipient fetus in the polyhydramniotic sac and ‘absent’ bladder in the anuric donor which is much smaller than the recipient (Fig. 25.2). If suspected, these pregnancies should be referred to tertiary fetal medicine centres for further management – first line management is usually laser surgery of inter-twin vascular placental anastomoses where the syndrome develops before 26 weeks’ gestation, other options include serial amnioreduction or elective delivery.
• Regular scans every 4 weeks after about 24 weeks until
acute hypotensive episodes as a result of haemorrhage from the live fetus into the dead fetoplacental unit (Box 25.1). • Higher rate of maternal pregnancy-related complications such as hyperemesis gravidarum, miscarriage, polyhydramnios, pre-eclampsia, anaemia and antepartum haemorrhage. • Higher rate of complications in labour – malpresentation, vasa praevia, cord prolapse, premature separation of placenta, cord entanglement and postpartum haemorrhage (PPH).
32 weeks, to monitor growth and fetal well-being, thereafter 2-weekly until 36 weeks. From 36 week onwards umbilical artery Doppler scan and amniotic fluid volume should be performed weekly (Box 25.2). • Anaemia should be looked for and treated vigorously. • Monitor for early signs of pre-eclampsia. Her 20-week detailed anomaly scan was normal. The 24-week scan showed normal growth of both the twins with normal liquor volume. The cervical length was found to be 41 mm which is within normal limits. Her husband works offshore.
Mrs Akinte wants some information on support groups. What support groups do you know of?
She is keen to know her likelihood of having a full-term pregnancy and how big the babies are likely to be when born?
• TAMBA: Twin and multiple birth association
See Table 25.1.
• The Multiple Birth Foundation • The UK Twin to Twin Transfusion Syndrome
Association
Table 25.1 Gestation periods for multiple births.
What kind of antenatal care can she expect during this twin pregnancy? • Detailed anomaly scan at 18–20 weeks. • Cervical length scan at 20–24 weeks to determine the
risk of preterm labour. Cervical length
