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multiple gestations

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الكلية كلية الطب     القسم  النسائية والتوليد     المرحلة 4
أستاذ المادة حنان خضير حسين       08/06/2017 12:46:46
TWINS AND HIGHER
MULTIPLE GESTATIONS

Multiple pregnancies consist of two or more fetuses. There are rare exceptions to this, such as twin gestations made up of a singleton viable fetus and a complete mole. Pregnancies with three or more fetuses are referred to as ‘higher multiples.

Risk factors for multiple gestations include assisted
reproductive techniques (both ovulation induction and in vitro fertilization (IVF)), increasing maternal age, high parity, black race and maternal family history.

Traditionally, the expected incidence was calculated using Hellin’s rule. Using this rule, twins were expected in 1 in 80 pregnancies, triplets in 1 in 802 and so on. The incidence of monozygotic or identical twins is generally accepted to be constant at 1 in 250, It is not influenced by race, family history or parity.
Non-identical or fraternal twins are dizygotic, having resulted from the fertilization of two separate eggs. Although they always have two functionally separate placentae (dichorionic), the placentae can become anatomically fused together and appear to the naked eye as a single placental mass. They always have separate amniotic cavities (diamniotic) and the two cavities are separated by a thick three-layer membrane (fused amnion in the middle with chorion on either side). The fetuses can be either same-sex or different sex pairings.
Identical twins are monozygotic – they arise from fertilization of a single egg and are always same-sex pairings. They may share a single placenta (monochorionic) or have one each (dichorionic)(DCDA). If dichorionic, the placentae can become anatomically fused together and appear to the naked eye as a single placental mass, as mentioned above. The vast majority of monochorionic twins have two amniotic cavities (diamniotic) but the dividing membrane is thin, as it consists of a single layer of amnion alone(MCDA). Monochorionic twins may occasionally share a single sac (monoamniotic).(MCMA).
Dizygotic twins may arise spontaneously from the release of two eggs at ovulation.
Monozygotic twins arise from a single fertilized
ovum that splits into two identical structures. The type of monozygotic twin depends on how long after conception the split occurs. When the split occurs within 3 days of conception, two placentae and two amniotic cavities result, giving rise to a dichorionic diamniotic (DCDA) pregnancy. When splitting occurs between days 4 and 8, only the chorion has differentiated and a monochorionic diamniotic (MCDA) pregnancy results. Later splitting after the amnion has differentiated leads to both twins developing in a single amniotic cavity, a monochorionic monoamniotic (MCMA) pregnancy. If splitting is delayed beyond day 12, the embryonic disc has also formed, and conjoined, or ‘Siamese’ twins will result.

All the physiological changes of pregnancy (increased
cardiac output, volume expansion, relative haemodilution, diaphragmatic splinting, weight gain, lordosis, etc.) are exaggerated in multiple gestations. This results in much greater stresses being placed on maternal reserves. The ‘minor’ symptoms of pregnancy also may be exaggerated.


Complications relavent to twin pregnancy:
• Miscarriage and severe preterm delivery.
• increase in perinatal mortality will be due to the excess of preterm delivery in monochorionic twins.
• Death of one fetus in a twin pregnancy.
With the more liberal use of early pregnancy scanning, it has been recognized that up to 25 per cent of twins may suffer an early demise and subsequently ‘vanish’ well before they would have previously been detected. After the first trimester, the intrauterine death of one fetus in a twin pregnancy may be associated with a poor outcome for the remaining co-twin. Maternal complications such as disseminated intravascular coagulation have been reported, but the incidence of this appears to be very low.
In dichorionic twins, the second or third trimester intrauterine death of one fetus may be associated with the onset of labour, although in some cases the pregnancy may continue uneventfully and even result in delivery at term. Careful fetal and maternal monitoring is required. By contrast, fetal death of one twin in monochorionic twins may result in immediate complications in the survivor. These include death or brain damage with subsequent neurodevelopmental handicap. Acute hypotensive episodes, secondary to placental vascular anastomoses between the two fetuses, result in haemodynamic volume shifts from the live to the dead fetus. The acute release of
vasoactive substances into the survivor’s circulation
may also play a role. Death or handicap of the co-twin occurs in up to 30 per cent of cases.
• Fetal growth restriction.
• Fetal abnormalities.
Compared to singletons, (dichorionic) twin pregnancies carry at least twice the risk of the birth of a baby with an
anomaly. In contrast, each fetus in a monochorionic twin pregnancy carries a risk for abnormalities that is four times that of a singleton.
• Chromosomal defects and twinning.
In twins, as in singletons, the risk for chromosomal abnormalities increases with maternal age.
Monozygotic twins arise from a single fertilized egg
and therefore have the same genetic make up. It is clear
that in monozygotic twin pregnancies, chromosomal abnormalities such as Down’s syndrome affect neither fetus or both. The risk is based upon maternal age. In dizygotic twins, the maternal age-related risk for chromosomal abnormalities for each individual twin remains the same as for a singleton pregnancy. Therefore, at a given maternal age, the chance that at least one of the twin pair is affected by a chromosomal defect is twice as high as for a singleton pregnancy. For example, a 40-year-old woman with a singleton pregnancy has a fetal risk of trisomy 21 of 1 in 100.
If she has a dizygotic twin pregnancy, the risk that
one fetus would be affected is 1 in 50 (1 in 100 plus
1 in 100).
• Complications unique to monochorionic twinning:
In all monochorionic twin pregnancies there are placental vascular anastomoses present, which allow communication between the two fetoplacental circulations.In approximately 15 per cent of monochorionic twin pregnancies, imbalance in the flow of blood across these arteriovenous communications results in twin-to-twin transfusion syndrome (TTTS). One fetus becomes overperfused and the other underperfused. The development of mild, moderate or severe TTTS depends on the degree of imbalance. The growth-restricted donor fetus suffers from hypovolaemia and becomes oliguric. As
fetal urine is the major component of amniotic fluid, this fetus develops oligohydramnios. The recipient fetus becomes hypervolaemic, leading to polyuria and polyhydramnios. There is also a risk of myocardial damage and high output cardiac failure.
The long-standing method of treatment has been amniocentesis every 1–2 weeks with the drainage of
large volumes of amniotic fluid. More recently, a small number of centres have used fetoscopically guided laser
coagulation to disrupt the placental blood vessels that connect the circulations of the two fetuses.
• Complications unique to monoamniotic twinning.
Monoamniotic twins share a single amniotic cavity,
with no dividing membrane between the two fetuses. They are at increased risk of cord accidents, predominantly through their almost universal cord entanglement.
• Complications in labour are more common with twin gestations. These include premature birth, abnormal
presentations, prolapsed cord, premature separation
of the placenta and postpartum haemorrhage.


Antenatal management
• Routine antenatal care for all women involves screening for hypertension and gestational diabetes. These conditions occur more frequently in twin pregnancies and there is also a higher risk of other problems (such as antepartum haemorrhage and thromboembolic disease); however, the management is the same as for a singleton.
• Determination of chorionicity ,this is done
most reliably by ultrasound in the late first trimester.
In dichorionic twins, there is a V-shaped extension
of placental tissue into the base of the inter-twin
membrane, referred to as the ‘lambda’ or ‘twin-peak’
sign. In monochorionic twins, this sign is absent and
the inter-twin membrane joins the uterine wall in a
T shape .Different-sex twins must be dizygotic and therefore dichorionic. In same-sex twins, two separate placentae mean dichorionic, although the babies may still be monozygotic.

• Screening for fetal abnormalities
The measurement of nuchal translucency at 12 weeks
gestation allows each fetus to have an individualized
assessment of risk.Monochorionic twins are monozygotic and therefore only one sample is needed for karyotyping.
Both amniocentesis and chorion villus sampling
(CVS) can be performed in twin pregnancies, but in
dichorionic pregnancies, it is essential that both fetuses
are sampled.
• Monitoring fetal growth and well-being
Measurement of symphysis–fundal height and maternal reporting of fetal movements are unreliable, as the individual contribution of each twin cannot be assessed. Monitoring for fetal growth and well-being in twins is principally by ultrasound.
In monochorionic twins, features of TTTS should be sought, including discordances between fetal size, fetal activity, bladder volumes, amniotic fluid volumes and cardiac size. It is reasonable to plan 4- to 6-weekly ultrasound scans in uncomplicated dichorionic twins. However, due to their higher background risk, fortnightly
ultrasound is appropriate in monochorionic pregnancies


Intrapartum management
• A twin CTG machine should be used for fetal monitoring and a portable ultrasound machine should be available during the delivery.
• It is essential that two neonatal resuscitation trolleys,
two obstetricians and two pediatricians are available
and that the special care baby unit and anaesthetist are
informed well in advance of the delivery.
• Epidural analgesia is recommended
• An abnormal fetal heart rate pattern in the
f rst twin may be assessed using fetal scalp sampling, as
for a singleton pregnancy. However, a non-reassuring
pattern in the second twin will usually necessitate
delivery by Caesarean section. The condition of the
second twin must be carefully monitored after the
delivery of the first twin, as acute complications such
as cord prolapse and placental separation are well
recognized.
• Vaginal delivery of vertex–vertex, After the delivery of the first twin, abdominal palpation should be performed to assess the lie of the second twin. If the lie is longitudinal with a cephalic presentation, one should wait until the head is descending and then perform amniotomy with a contraction. If contractions do not ensue within 5–10 minutes after delivery of the first twin, an oxytocin infusion should be started.
• Delivery of vertex–non-vertex,
If the second twin is a breech, the membranes can
be ruptured once the breech is fi xed in the birth canal.
A total breech extraction may be performed if fetal
distress occurs or if a footling breech is encountered,
• Where the second fetus is transverse, external cephalic version can be successful,if not an internal podalic version can be undertaken
• Non-vertex first twin
When the first twin presents as a breech, clinicians
usually recommend delivery by elective Caesarean
section.
• The risk of postpartum haemorrhage is increased in twin pregnancies due to the larger placental site and
uterine over-distension. For that reason, all multiple
gestations should have an intravenous line and blood
grouped and saved during labour. Management is generally no different from that of postpartum haemorrhage complicating singleton delivery
• Higher multiples, Caesarean section is usually
advocated for delivery due to the difficulties of
intrapartum fetal monitoring.


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