The incidence of twins is 3 in 200 pregnancies; triplets 1 in 10,000 pregnancies.

The incidence is increasing alongside the increased uptake of IVF therapy.


Associated Factors


    • Family history (if dyzygotic twins)
    • Increased maternal age
    • IVF therapy and induced ovulation
    • Previous multiple pregnancy


        Types of twin pregnancy


        MONOZYGOTIC TWINS (MZ) = "identical twins"

        Normally the fertilised egg divides into two, then four cells, then eight and so forth.

        However, with MZ twins, the egg completely splits into two separate bodies after the two-cell stage resulting in a pregnancy that will go on to produce two fetuses.

        If this occurs before day 3 of the pregnancy, then the fetuses will have different placentas (dichorionic) and different amnions (diamniotic) [DCDA twins].

        If the division occurs between days 4-8 (~70%) then the twins will share the same placenta (monochorionic) but different amnions (diamniotic) [MCDA].

        Rarely, the division occurs later on and can lead to monochorionic monoamniotic twins [MCMA]. 


        DIZYGOTIC TWINS (DZ) = "non-identical twins"

        This is where two different oocytes released during the same menstrual cycle, are fertilised by different sperm. It accounts for ~2/3 of all multiple pregnancies. The fetuses may therefore be of different sex. This scenario can allow for the two oocytes being fertilised by different fathers. 




        Very rarely results from the release of 3 oocytes.

        In this scenario, usually 2 oocytes are released as happens with dizygotic twins. However, one of the 2 fertilised oocytes then goes on to divide post-fertlisation (as happens with monozygotic twins) to produce a 3rd embryo.


        Antenatal management


        Mothers of multiple pregnancies should be under consultant-led care. Delivery should take place in an obstetric unit with 24h obstetric theatre facilities.



        The mother may complain of hyperemesis gravidarum. In multiple pregnancy, this may be excessive.



        The mother will be large for dates and after 30-32 weeks, multiple fetal poles may be palpated.



        Ultrasound scan (USS) at 7-8 weeks will show multiple embryos. An expert can determine whether the twins are MZ or DZ. 

        USS at 28, 32 and 36 weeks.

        Monochorionic twins: fortnightly USS from 12 weeks to check for twin-twin transfusion syndrome and IUGR.

        The mother will require extra iron and folic acid supplements.

        Regular blood checks should take place to check for anaemia.




          • Perinatal mortality increased 4-fold
          • Increased incidence of preterm labour and miscarriage
          • Increased incidence of congenital abnormalities
          • Intrautertine growth restriction (IUGR)
          • Pre-eclampsia more common
          • Maternal anaemia more common
          • Antepartum and postpartum haemorrhage more common
          • Malpresentation


            Intrapartum management


            Delivery should take place in an obstetric unit. An obstetrician experienced in multiple pregnancies should be present. Continuous monitoring should be initiated.

            An anaesthetist and paediatrician should be kept informed. Ideally, one paediatrician for each baby should attend delivery.

            However, avoid having too many people in the delivery room as this can scare the mother.


            Elective delivery at 37-38 weeks is common. 12% of twins deliver before 32 weeks.

            Cord prolapse is more common so be vigilant.

            Do NOT give Syntometrine after delivery of Twin 1.


            Uterus usually starts contracting within 5 minutes of delivery of Twin 1, and twin 2 is usually delivered within 20 minutes of Twin 1. 


            Give syntometrine after delivery of Twin 2 and continue oxytocin infusion for 1 hour post-delivery.


            Deliver placentae after Twin 2. 



            COLLIER, J., LONGMORE, M., TURMEZEI, T. & MAFI, A.R. 2009. Oxford Handbook of Clinical Specialities. 8th edition. Oxford, UK: Oxford University Press.

            CRUIKSHANK, D.P. 2007. Intrapartum Management of Twin Gestations. Obstetrics & Gynecology, 110(3), pp. 1167-1176.

            HAMILTON-FAIRLEY, D. 2009. Obstetrics and Gynaecology. 3rd edition. Oxford, UK: Wiley-Blackwell Publishing.

            IMPEY, L. & CHILD, T. 2008. Obstetrics & Gynaecology. 3rd edition. Oxford, UK: Wiley-Blackwell Publishing.




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