The commonest litter size in the human species is one. Other multiples are rare and vary with racial characteristics inside the species (e.g. there is a higher incidence in West Africa than in Europe).
The differences between actual and theoretical figures are probably due to the reducing birth rate, more births to older women and fewer to those of higher parity; (both having increased multiple pregnancy rates). Higher rates in the UK are now due to ovarian stimulation and assisted conception with a trebling of triplets in IVF.
Types of Multiple Pregnancies
Monovular twins are produced from one ovum fertilized by one sperm. After the two-cell division instead of going into the four-cell stage, the blastomere divides into two separate cell bodies which go on to two individuals. Thus there is common chromatin material; sex and physical characteristics will be the same, producing identical twins.
Binovular twins are from two separate ova fertilized by two different sperms. These ova are shed in one menstrual cycle and most likely to be fertilized after one intercourse although they can be at separate times with different fathers. The two blastomeres develop separately and have different chromatin material. They can, therefore, be of different sexes having no more in common than any other members of the same family. They are non-identical twins. Early ultrasound can help differentiation.
Monovular twins have an incidence of 3 or 4:1000 worldwide and there is only a slight familial tendency. Binovular twins may have a family history on the maternal side. It is these that account for racial and maternal age variations. Binovular twins are more common than monovular ones (4:1).
Binovular twins are commoner if:
- Maternal family history of non-identical twins.
- Over 35 years.
- After replacement of two, three or more fertilized ova at in vitro fertilization.
Differentiation of twins
1 Sex. If of different sexes, obviously binovular. If of one sex, maybe either.
2 Placenta. If two separated placentae, will be binovular; if one placenta may be monovular or binovular. Check septum between sacs by peeling amnions from each other
3 Blood groups. If doubt in dichorionic types, check the ABO, Rh, Duffy, Kell and MNS.
4 Fingerprints. If different, binovular.
5 DNA fingerprinting with probes identifying about 60 dispersed sequences of variable size.
|Septum||Placenta type||Twin type|
|(3) Amnion only||Diamniotic|
|(2) Amnion and chorion||Diamniotic|
|Binovular or Monovular|
|(1) No common septum||Diamniotic|
Diagnosis of twins
• Suspicion on family history especially maternal non-identical twins.
• Suspicion on past obstetric history of twins.
• Suspicion from excessive vomiting in early pregnancy.
Examination from 20 weeks onwards shows uterus bigger than expected. At first, a lot of limbs are felt and later, about 30–32 weeks, more than two separate poles determined (e.g. two heads and one breech).
Ultrasound at 6–7 weeks may show two or more sacs. The embryos can be seen in these at 7–8 weeks. The differentiation of mono from binovular can often be made by expert examination of the dividing membranes.
Commonly one of a pair of twins diagnosed early does not develop and is absorbed: the vanishing twin syndrome.
Without ultrasound, twins may not be diagnosed until delivery on rare occasions. While embarrassing to the attendants, this usually does not affect the second twin unless Syntometrine was given inadvertently at the birth of the first baby. This could jeopardize the O2 supply to the second twin and so his or her delivery should be expedited.
Management of twins
Complications in pregnancy
1 Miscarriage is more frequent.
2 Preterm labour commoner (50% before 37 weeks).
3 Pre-eclampsia commoner (¥3).
4 Risk of anaemia increased.
• Iron deficiency.
• Folic acid deficiency.
5 Polyhydramnios commoner (¥10 with monovular twins).
6 Risk of APH increased.
• Abruptio placentae.
• Placenta praevia.
Management in pregnancy
1 Diagnose early by bearing it in mind (one only diagnoses what one thinks about); often ultrasound will give the result before clinical suspicion.
2 Give extra iron and folic acid supplements and see that the woman takes them.
• Check blood more often for haemoglobin levels.
Complications in labour
1 Delay in delivery of the second twin is associated with a higher mortality.
2 PPH is more common.
3 Prolapse of umbilical cord is more common.
4 Mechanical collision of leading parts (or locking of a breech–cephalic) as they both enter the pelvis. This is very rare.
Management in labour
1 Always plan for hospital delivery.
2 Ensure the first twin is longitudinal. Commonest combinations of presentations show that both twins lie longitudinally 90% of the time and the first twin is a cephalic presentation in 80%. Non-cephalic presentations are common if early preterm labour. If the first twin is transverse, do a Caesarean section.
3 Check for cord prolapse when membranes rupture (often early in labour).
4 Progress is usually uneventful. Monitor both fetal hearts and have an i.v. drip running.
5 An epidural anaesthetic is useful for it allows rapid anaesthesia for any manoeuvres that may be required for the second twin in the second stage.
6 Deliver the first twin appropriately. Have an anaesthetist and a paediatrician in the labour ward. Make sure that nobody inadvertently gives Syntometrine to the mother at this point.
7 Clamp cord of the first twin and divide. Hand baby to competent assistant or paediatrician in case resuscitation is required.
8 Immediately check the lie of the second twin. If longitudinal, check presenting part. If oblique or transverse, convert to longitudinal.
• External version —usually easy for uterus is lax, or
• Combined external and internal version —rupture membranes and bring a leg of the fetus down through the cervix. This produces an incomplete breech presentation but it is at least longitudinal.
9 The second twin is best delivered within 20 minutes of the first. Usually uterus starts contracting again about 5 minutes after first delivery. If it does not do so spontaneously, use i.v. oxytocin augmentation. Very little is needed. Rupture membranes of second sac and deliver appropriately.
10 Give Syntometrine with delivery of the second twin and continue oxytocin infusion for another hour.
11 Deliver placentae as soon as uterus is concontracted after delivery of the second twin, for retained placentae and PPH are common.
• The complications of pregnancy, e.g. pre-eclampsia.
• The complications of delivery, e.g. anaesthesia and PPH.
• Risks to the first twin are twice, and to the second twin, about three times those of single births.
Causes of death:
(a) Immaturity. Especially
(b) Hypoxia. the second
(c) Risks of operative delivery. twin.
• Neonatal risks are jaundice or anaemia following intrauterine shunting of blood inside the placenta leading to twin-to-twin transfusion in monovular twins.
Rarely due to tri-ovulation:
• Usually binovular twins with one fertilized egg dividing into two individuals or assisted conception.
• Usually born at an even more immature stage than twins and have double the risks.
• The complications and management are as for twins. Because of the immaturity of the fetuses, delivery is commonly by Caesarean section.