Multiple Pregnancy

Chapter 16 Multiple Pregnancy


The term ‘multiple pregnancy’ is used to describe the development of more than one fetus in utero at the same time.



Twin pregnancy



Types of twin pregnancy


Twins will be either monozygotic (MZ) or dizygotic (DZ) (Box 16.1). Of all twins born in the UK, two-thirds will be DZ and one-third MZ.



Box 16.1 Monozygosity and dizygosity



Monozygotic or uniovular twins




• Also referred to as ‘identical twins’


• Develop from the fusion of one oocyte and one spermatozoon, which after fertilisation splits into two


• Are of the same sex and have the same genes, blood groups and physical features such as eye and hair colour, ear shapes and palm creases; may be of different sizes and sometimes have different personalities



Dizygotic or binovular twins




• Also referred to as ‘non-identical twins’


• Develop from two separate oocytes that are fertilised by two different spermatozoa


• Are no more alike than any brother or sister and can be of the same or different sex


Superfecundation is the term used when twins are conceived from sperm from different men if a woman has had more than one partner during a menstrual cycle. It is not known how often this happens, but if suspected then paternity can be checked by DNA testing.


Superfetation is the term used for twins conceived as the result of two coital acts in different menstrual cycles. This is thought to be very rare.



Determination of zygosity and chorionicity


Determination of zygosity means determining whether or not the twins are identical.



image In about one-third of all twins born this will be obvious, as the children will be of a different sex.


image At birth, identical twins tend to have a greater weight variation than non-identical ones.


image In approximately two-thirds of identical twins a monochorionic placenta will confirm monozygosity. If the babies have a single outer membrane, the chorion, they must be MZ.


image In one-third of identical twins the placenta will have two chorions and two amnions, and either fused placentae or two separate placentae (dichorionic); this situation is indistinguishable from that in non-identical twins. It occurs when the fertilised oocyte splits within the first 3 or 4 days after fertilisation and while it is still in the uterine tube.


image When these entities are seen on an early scan they appear as two separate placentae and are dichorionic and diamniotic, exactly the same as non-identical twins.


image In about two-thirds of cases the division occurs up to approximately 10–12 days after fertilisation; these will be monochorionic and diamniotic.


image Monoamniotic twins occur in about 1% of cases, when the embryo divides after 12 days.


Monochorionic twin pregnancies have a 3–5 times higher risk of perinatal mortality and morbidity than dichorionic twin pregnancies.



Zygosity determination after birth


The most accurate method of determining zygosity is to compare DNA. DNA can be extracted from cells taken from a cheek swab from inside the mouth. Specific genetic markers extracted from different chromosomes are compared and the results are up to 99.99% accurate.


Zygosity determination should be routinely offered to all same-sex twins for the reasons listed in Box 16.2.



Box 16.2 Reasons for determining zygosity




• Most parents will want to know whether or not their twins are identical


• If parents are considering further pregnancies, they will want to know the likelihood of having twins again:


image DZ twins tend to run in families and the increased likelihood is approximately five-fold

image MZ twins do not run in families and the likelihood does not change

• It will help the twins in establishing their sense of identity; it will influence their life and family relationships


• The information is important for genetic reasons: not just the monogenic disorders but with any serious illness later in life


• Twins are frequently asked to be involved in research where knowledge of zygosity is essential



Diagnosis of twin pregnancy


This is usually through ultrasound examination. Diagnosis can be made as early as 6 weeks into the pregnancy, or later at the routine detailed structural scan between the 18th and 20th weeks. A family history of twins should alert the midwife to the possibility of a multiple pregnancy. Occasionally (1 in 12 000 live births), one fetus may die in the second trimester and become a fetus papyraceous, which becomes embedded in the surface of the placenta and expelled with the placenta at delivery.



Abdominal examination


See Box 16.3.



Box 16.3 Abdominal examination in the diagnosis of multiple pregnancy



Inspection




• The size of the uterus may be larger than expected for the period of gestation, particularly after the 20th week. The uterus may look broad or round


• Fetal movements may be seen over a wide area, although the findings are not diagnostic of twins


• Fresh striae gravidarum may be apparent


• Up to twice the amount of amniotic fluid is normal in a twin pregnancy but polyhydramnios is not an uncommon complication of a twin pregnancy, particularly with monochorionic twins



Palpation




• The fundal height may be greater than expected for the period of gestation


• The presence of two fetal poles (head or breech) in the fundus of the uterus may be noted; multiple fetal limbs may also be palpable


• The head may be small in relation to the size of the uterus


• Lateral palpation may reveal two fetal backs or limbs on both sides


• Pelvic palpation may give findings similar to those on fundal palpation, although one fetus may lie behind the other and make detection difficult


• Location of three poles in total is diagnostic of at least two fetuses



Auscultation




• Hearing two fetal hearts is not diagnostic; however, if simultaneous comparison of the heart rates reveals a difference of at least 10 beats per minute, it may be assumed that two hearts are being heard



The pregnancy


A multiple pregnancy tends to be shorter than a single pregnancy. The average gestation for twins is 37 weeks, for triplets 34 weeks, and for quadruplets 33 weeks.



Effects of pregnancy



Exacerbation of common disorders


More than one fetus and the higher levels of circulating hormones often exacerbate the common disorders of pregnancy. Sickness, nausea and heartburn may be more persistent and more troublesome than in a singleton pregnancy.



Anaemia


Iron deficiency and folic acid deficiency anaemias are common. Early growth and development of the uterus and its contents make greater demands on the maternal iron stores; in later pregnancy (after the 28th week) fetal demands may lead to anaemia. Routine prescription of iron and folic acid supplements is not necessary.



Polyhydramnios


This is particularly associated with monochorionic twins and with fetal abnormalities. Polyhydramnios will add to any discomfort that the woman is already experiencing. Acute polyhydramnios can lead to miscarriage or premature labour.



Pressure symptoms


Impaired venous return from the lower limbs increases the tendency to varicose veins and oedema of the legs. Backache is common and the increased uterine size may also lead to marked dyspnoea and to indigestion.



Other effects


There can be an increase in complications of pregnancy.



Antenatal screening




image Nuchal translucency for Down syndrome is accurate only if performed between 11 and 13 weeks.


image Serum screening is not usually performed in multiple pregnancy, as results are too complex to interpret.


image Chorionic villus sampling (CVS) is not usually recommended in multiple pregnancy, as loss rates are high.


image Amniocentesis can be performed in twin pregnancies, usually between 15 and 20 weeks. It should be performed in a specialist fetal medicine unit. Most obstetricians prefer to do a dual needle insertion so there is no chance of contamination between the two sacs.


image Chorionicity should be determined in the first trimester.


image All MZ twins should have echocardiography performed at approximately 20 weeks’ gestation, as there is a much higher risk of cardiac anomalies in these babies.



Ultrasound examination




image Monochorionic twin pregnancies should be scanned every 2 weeks from diagnosis to check for discordant fetal growth and signs of twin-to-twin transfusion syndrome (TTTS).


image Dichorionic twin pregnancies should be scanned at 20 weeks for anomalies, and then usually every 4 weeks.

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Related posts:

  1. The Placenta
  2. Prolonged Pregnancy and Disorders of Uterine Action
  3. Obstetric Emergencies
  4. Problems of Pregnancy

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Jul 11, 2016 | Posted by in MIDWIFERY | Comments Off on Multiple Pregnancy

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