MULTIPLE PREGNANCY

Introduction

The natural incidence of twinning has a large geographical variation,ranging from 54/100

in the UK to 4/1000 in japan.this difference is almost entirely due to variations in the rate of non-

identical twins remains remarkably constant at around 3/100.in defeloped countries,the actual

incidence of twin pregnancies is significantly greater than the natural incidence,due to in vitro

fertilization and ovulation induction techniques.around 25% of twin pregnancies ,50-60% of

triplet pregnancies,and 75% of quadruplet pregnancies are aresult of assisted reproduction

techniques.

Overall,the perinatal mortality in twin pregnancies is four to five times higher than for

singleton pregnancies,largely because of preterm delivery,fetal growth restriction,twin-twin

transfusion syndrome (TTTS),and a slightly increased incidence of congenital

malformations.perinatal mortality rates rise exponentially with fetal number in higher-order

pregnancies.the outcome of any multiple pregnancy is also significantly affected by its

chorionicity (wheter each fetus has its own or shares a placenta) ( fig.36.1).

The nature of twinning and chorionicity

‘Zygosity’ refers to whether the twins have come from the same ovum or from different

ova-in other words whether they are identical or non-identical.’chorionecity’ refers to the

number of placentae (fig.36.2).

Dizygotic twinning (non-identical)

Dizigotic twins account for approximately 70% of twins.this process accurs when two

ova are fertilized and implant separately into the deciduas.each developing embryo will form its

own outer chorion (chorionic membrane and placenta) and its own inner amniotic

membrane.dizygotic twin pregnancies are described as dichorionic and diamniotic.

Monozygotic twinning (identical)

Monozygotic twins (30% of twins) are derived from the spilitting of a single ebryo,and

the exact configuration of placentation depends on the age of the embryo when the split occurs

(fig.32.2).A split that occurs at or before the eight-cell stage (3 days post-fertilization) will occur

before the outer chorion has differentiated and will therefore give rise to two separate embryos

that will each proceed to form their own chorion.These twin pregnancies,like dizygotic

twins,will therefore be diamniotic and dichirionic.Embryosplitting at the blastocyst stage (4-8

days post-fertilization) will accur after the chorion has started to differentiate and therefore the

fetuses will share an outer chorion (placenta and outer chorionic membrane).this is the more

common form of monozygotic twinning.division of the ebryo at between 8 and 14 days will

result in the inner amniotic cavity and membrane being shared (monochorionic monoamniotic

twins).Splitting beyond 14 days following fertilization is extremely rare,giving rise to conoined

twins (fig.36.3;table 36.1).

In monochorionic twins the shared placental mass inevitably contains a number of

vascular anastomoses between the two fetal-placental circulations.The number and nature of

these vascular connections places monochorionic twins at risk of specific complications and an

increased perinatal loss and morbidity rate.

Chorionicity determination is therefore essential to allow risk stratification (table

36.2),and has key implacations for prenatal diagnosis and antenatal monitoring.it is most easily

determined in the first or early second trimester by ultrasound:

Widely separated first-trimester sacs or separate placentae are dichorionic.

Those with a ‘lambda’ or ‘twin-peak’ sign at the membrane insertion are dichorionic

(fig.36.4A).

Those with a ‘T’ sign at the membrane insertion are monochorionic (fig.36.4B).

Different-sex fetuses are always dichorionic (and dizygous).

Table 36.1

Chorionicity in monozygous twins

Timming of

embryonic separation

after fertilization

Number of chorions Number of amniotic

sacs

Percentage of

monozygous twins

<4 days Dichorionic Diamniotic 30%

4-8 days Monochorionic Diamniotic 66%

8-4 days Monochorionic monoamniotic 3%

>14 days Monochorionic

(conjoined)

Monoamniotic <1%

Fig 36.1 Trichorionic placenta

Fig.36.2 diagram of chorionicity. Monozygotic pregnancies may form any of the following combinations

depending on the gestation of embryo division : (A) dichorionic diamniotic; (B) monochorionic diamniotic; (C)

monochorionic monoamniotic; (D) conjoined twins.

Fig. 36.3 conjoined twins. Dignosed at 12 week’s gestation. This is a cross-sectional view through the thoraces of

both of the twins. In view of the shared cardiac structures, termination was offered.

Fig. 36.4 Dichorionic twins-lambda sign (A), Monochorionic twins-no lambda sign (B). the two amniotic

membranes form a T-sign’as they join the placenta.

Fig. 36.5 Monochorionic twins demonstrating twin twin transfusion syndrome (oligohydramnion-

polyhtdramnios sequence)

Maternal complications

The incidence of all maternal complications is increased in multiple pregnancy.

Hyperemesis

The increased placental mass,and therefore increased maternal circulating human chorionic

gonadotrophin (hCG) concentration,is associated with an increased and earlier incidence of

hyperemesis.

Anaemia

There is a slight increase in the incidence of anaemia associated with multiple

pregnancy,which is not completely explained by a haemodilutional effect of the increased

plasma volume.The extra iron and folate requirement may justify routine supplementation.

Antepartum haemorrhage

Placenta praevia is more common with multiple gestation as a result of the larger placental

surface.The management of this condition in multiple pregnancy is similar to that of a

singleton pregnancy.Placental abruption also appears to be commoner in twin pregnancies.

Pre-eclampsia

The incidence of pre-eclampsia in twin pregnancy is three to four times greater than that in

singleton pregnancies.It tends to develop earlier and may be more severe.

The mother is also at increased risk of gestational diabetes,general discomfort,varicose

veins and dependent oedema,delivery trauma,caesarean section,postpartum haemorrhage

and breastfeeding challenges.

Table 36.2

Fetal loss by chorionicity

Dichorionic Monochorionic

Fetal loss before 24 weeks 1.8% 12.2%

Fetol loss after 24 weeks 1.6% 2.8%

Delivery before 32 weeks 5.5% 9.2%

The high early fetal mortality in monochorionic pregnancy before 24 weeks is probably due

largely to severe early-onset twin to twin transfusion sequence (see below).

Fetal complications

Structural defects

The incidence of structural fetal abnormality is no different per fetus in a dichorionic pregnancy

compered to a singleton pregnancy,but it is two- to threefold greater with monochorionicity.The

mother therefore will have a two-to sixfold increased risk of carryng a fetus with a structural

abnormality.In monochorionic twins it is thought that it is the processof embryo division which is

inherently teratogenic.Characteristic abnormalities include cardiac defects,neural tube and other

CNS defects,and gastrointestinal atresia.It is appropriate to offer all those with multiple pregnancies

a detailed mid-trimester ultrasound scan.The abnormalities are usually confined to one

twin(i.e.nonconcordent);for example,if there is a neural tube defect in one twin,the other twin is

normal in 85-90% of cases.Selective termination with intracardiac KCL is possible in dichorionic

pregnancies only,and is most safely carried out before 16-20 weeks.The procedure,however,carries

a 5% risk of miscarriage of both twins.In monochorionic twins,specialized cord occlusion

techniques may be considered,but carry an increased risk of loss to the other twin due to the

increased invasiveness of this procedure.

Chromosomal abnormalities

These are usually discordant in dizygotic twin and almost always concordant in monozygotic

twins.The maternal agerelated risk for carrying a fetus with down syndrome is there fore

approximately doubled in dichorionic twin pregnancies.Maternal serum screening for trisomy 21

performs poorly in twin pregnancy.Nuchal translucency measurement (without biochemistry) is a

more useful screening test.If invasive diagnostic testing is indicated,amniocentesis of each amniotic

sac is required in dichorionic pregnancies,and care must be taken to document which sample has

come from which sac.

Chorionic villous sampling is not usually appropriate for twin pregnancies as it difficult to be

sure that both placentae have been sampled, particularly if they are lying close to gether.

Premature birth

Twins typically deliver by 37-38 weeks’ gestation. Twins account for 25% of all premature

births despite accounting for only 2% of births per year. Preterm delivery is higher in

monochorionic compared to dichorionic twins (table 36.2). Increased uterine distension, early

myometrial contractility and TTTS may be causative factors in premature labour in multiple

pregnancy. At present there is no known effective treatment to prevent premature labour.

Women should be advised to present early with any symptoms of suspected preterm labour so

that corticosteroids can be administered to accelerate fetal lung maturation.

Intrauterine growth restriction

Twins typically reflect singleton size charts until 28-30 weeks’ gestation and then growth slows.

Approximately 30% of twins are small for gestational age by singleton standards and a

significant in the growth of one twin compared to the other is seen in 12% of pregnancies.

Placental dysfunction underlies intrauterine growth restriction in twin pregnancies as in singleton

pregnancies. Abdominal palpation is not reliable to monitor fetal growth in multiple pregnancy.

Serial ultrasound be performed to measure fetal abdominal circumferences. If diagnosed,

intrauterine growth restriction requires increased surveillance of fetal well-being with umbilical

artery Doppler and cardiotocography (CTG) monitoring, so that delivery can be optimally timed.

Monochorionic twins are at increased risk of intrauterine growth restriction, and require a lower

threshold for delivery owing to the adverse consequences of a single intrauterine death in these

twins.

Twins with one fetal death

First-trimester intrauterine death in a twin has not been shown to have adverse consequences for

the survivor. This probably also holds true for the early second trimester, but loss in the late

second or third trimester commonly precipitates labour such that 90% will have delivered both

twins within 3 weeks of the loss. Prognosis for a surviving dichorionic fetus is then influenced

primarily by its gestation. When a monochorionic twin dies in utero, however, there are

additional risks of death (approximately 20%) or cerebral damage (approximately 25%) in the

co-twin as a result of the shared fetal-placental circulations. As these are probably related to

acute hypotension in the co-twin at the time of the other’s death, early delivery of the surviving

twin is unlikely to improve its outcome and may compound morbidity if performed at a

premature gestation.

Antenatal problems specific to monochorionic twin

pregnancies

Twin-twin transfusion syndrome (TTTS)

This complicates 10-15% monochorionic multiple pregnancies and accounts for around 15% of

perinatal mortality in twins. In this condition there is a net blood flow from one twin to the other

through arterial to venous anastomoses in the shared placenta. The circulation of the recipient

becomes hyperdynamic, with the risk of high-output cardiac failure and polyhydramnios.

Conversely, the donor develops oliguria and oligohydramnios and often suffers growth

restriction (Figs.36.5, 36.6). The ultrasound finding of the oligohydramnios/polyhydramnios

sequence is the key to establishing an antenatal diagnosis.

Without treatment TTTS is associated with a greater than 80% pregnancy loss rate. Two

interventions have proven useful: serial amniodrainage and laser ablation of the causative

placental vascular anastomoses. Evidence has emerged that laser ablation is the most effective

intervention (70% survival vs 50% survival with amnioreduction). Laser therapy is also

associated with a lower rate of significant neurological morbidity in surviving twins compared to

amnioreduction (5% vs 15%).

Fig.36.6 twin-twin transfusion sequence. These monochorionic twins were born at 37 weeks’ gestation. Although

their weight were almost identical, there was significant oligohydramnions around the recipient. (A) Pre-transfusion.

(B) Post-transfusion. (with permission).

Monoamniotic twins

Twins who occupy a single amniotic sac, are at risk of cord entanglement in utero. Frequent

CTG monitoring is required once they reach viability. Delivery is indicated if cord compression

is subsequently diagnosed. Delivery is otherwise electively planned for 32 weeks’ gestation.

Delivey should be performed by caesarean section, as the risk of a cord accident is particularly

high during labour.

Twin reversed arterial perfusion sequence

If the heart of one monochorionic twin stops, it may continue to be partially perfused by the

surviving twin if large fetal arterial-to-arterial anastomoses axist in the shared placenta. The dead

twin undergoes atrophy of its upper body and heart due to the especially poor oxygenation of

these tissues, and becomes what has been described as an’acardiac monster’. The condition is

very rare and there is a high incidence of mortality in the donor twin due to intrauterine cardiac

failure and prematurity. Cord ligation has been used with success in isolated cases.

Management of pregnancy

Initial visit

As many as 10% of twin pregnancies diagnosed in the first trimester will proceed only as

singleton pregnancies and

Subsequent visits

These are ideally performed at a dedicated twins clinic and timed to coincide with the ultrasound

assessments ,the schedule of which will depend on chorionicity.

Monochorionic twins:

Every 2 weeks from 16 to 24 weeks to survey for TTTS.

Detailed structural survey at 18 weeks’ gestation.

Detailed fetal cardiac scan at 20-22 weeks’ gestation.

Every 2 weeks from 24 weeks for fetal growth assessment.

Dichorionic twins:

Detailed structural survey at 18 weeks’ gestation.

Every 2-4 weeks from 24 weeks for fetal growth assessment.

The mother should be monitored for complications such as pre-eclampsia and

anaemia.Discussions abouth the risk and management of premature delivery and fetal growth

problems are useful at 22-24 weeks’ gestation.In uncomplicated pregnancies,discussion around

mode of delivery and management of twin labour is usuful at 32 weeks’ gestation,when fetal

presentations are unlikely to change.Tailored parencraft advice or classes is worthwhile.

Management of twin delivery

Presentations at term are typically:

Cephalic/cephalic (40%)

Cephalic/breech (40%)

Breech/cephalic (10%)

Other,e.g. transverse (10%).

It is common practice to induce labour at 38-40 weeks in those who are suitable for

vaginal delivery,and to carry out a caesarean section at 38 weeks in those who are not.In general

with twins,providing the presentation of the first twin is cephalic,the balance of current evidence

would suggest that vaginal birth is appropriate.Significant growth discordance may be a reason

to consider caesarean section.If the labour is preterm (<34 weeks),many clinicians would also

consider delivery by caesarean section.

The first stage of labour is managed as for singleton pregnancies and care should be taken

to ensure that both twins are being monitored with CTG,rather than one twin twice.This is best is

best achieved by monitoring twin I with a fetal scalp electrode and twin II abdominally.

An experienced obstetrician,an anaesthetist,two paediatricians and two midwives should

be present for delivery,and,if not already required,a syntocinon infusion should be ready in case

uterine activity decreases after delivery of the first twin.After delivery of the first twin,it is often

helpful to have someone ‘stabilize’ the lie of second twin to longitudinal by abdominal palpation

while a vaginal examination is performed to assess the station of the presenting part.A

portable,ultrasound machine is helpful to confirm the lie and presentation of the second twin.The

membranes of twin II should not be broken until the presenting part has descended into the

prlvis.If twin II lies transversely after the delivery of twin I,external chapalic or breech version is

appropriate.If the lies is still transverse (particularly likely if the back is towards the fundus),the

choice is between breech extraction (gentle continuous traction on one or both feet through intact

membranes) and caesarean section.The CTG of twin II should be carefully monitored throughout

and delivery expedited if suspected fetal distress is observed.

A maternal epidural is useful in the management of twin labour owing to the increased

risks of obstetric intervention,particularly assisted delivery of twin il.

Triplets and higher multiples

In these cases,the perinatal mortality is high,mostly because of the high risk of premature

labour,and it may be appropriate to discuss reducing the number of fetuses to twins at 12-14

weeks’ gestation.With quadruplets or higher-order pregnancies,there is likely to be a greater

chance of at least one or two survivors if fetal reduction is carried out,despite the miscarriage

risk associated with the procedure itself.For triplets,the situation is less clear.The emotional and

ethical problems associated with these decisions are considerable.

Triplets and higher-order multiple pregnancies require intensive antenatal care.These

pregnancies are best delivered by caesarean section due to the inability to effectively monitor all

fetuses in labour and the higher risk of fetal malpresentation.

Key points

It is essential to establish chorionicity early to help advise about prenatal diagnosis and

stratify subsequent care.Prenatal screening should only be under taken after careful

discussion of its implications.

Monochorionic pregnancies are at further increased risk of preterm delivery and

intrauterine growth restriction.They also have specific additional risks such as twin-twin

transfusion syndrome and loss of co-twin complications.