diagnosis and assessment of multiple pregnancy

Last reviewed 04/2022

The most important clinical indicators of multiple pregnancy are the finding of increased uterine size or growth rate, and, in later pregnancy, the finding of multiple fetal parts on palpation. Definitive diagnosis is by ultrasound which may diagnose twins at 7 weeks or earlier. A raised serum AFP occurs in 75% of twins.

Other clinical indicators:

  • early symptoms of pregnancy such as morning sickness are more pronounced due to increased hCG concentrations
  • excessive fetal movements - particularly noticeable if previously had singleton pregnancy
  • anaemia more likely if compensatory dietary adjustments not made - both iron-deficiency and megaloblastic anaemia
  • large size of uterus may increase incidence of problems such as discomfort, shortness of breath, backache, oedema, haematuria, varicose veins
  • multiple fetal hearts may be heard on examination - different places, different rates

On ultrasound scan, separate gestation sacs can be identified at 7 weeks or earlier; separate fetal bodies are discernible from the 12th week; and separate heads from the 14th week.

NICE have given guidance regarding the diagnosis and assessment of multiple pregnancies (1)

Determining gestational age and chorionicity

  • offer women with twin and triplet pregnancies a first trimester ultrasound scan when crown-rump length measures from 45 mm to 84 mm (at approximately 11 weeks 0 days to 13 weeks 6 days) to estimate gestational age, determine chorionicity and screen for Down's syndrome (ideally, these should all be performed at the same scan)

  • determine chorionicity at the time of detecting twin and triplet pregnancies by ultrasound using the number of placental masses, the lambda or T-sign and membrane thickness

  • assign nomenclature to babies (for example, upper and lower, or left and right) in twin and triplet pregnancies and document this clearly in the woman's notes to ensure consistency throughout pregnancy

  • use the largest baby to estimate gestational age in twin and triplet pregnancies to avoid the risk of estimating it from a baby with early growth pathology.

Monitoring for intrauterine growth restriction

  • estimate fetal weight discordance using two or more biometric parameters at each ultrasound scan from 20 weeks
    • aim to undertake scans at intervals of less than 28 days
    • consider a 25% or greater difference in size between twins or triplets as a clinically important indicator of intrauterine growth restriction and offer referral to a tertiary level fetal medicine centre

Screening for structural abnormalities

  • offer screening for structural abnormalities (such as cardiac abnormalities) in twin and triplet pregnancies as in routine antenatal care
  • consider scheduling ultrasound scans in twin and triplet pregnancies at a slightly later gestational age than in singleton pregnancies and be aware that the scans will take longer to perform
  • allow 45 minutes for the anomaly scan in twin and triplet pregnancies
  • allow 30 minutes for growth scans in twin and triplet pregnancies.

Monitoring for feto-fetal transfusion syndrome

  • monitoring for feto-fetal transfusion syndrome should not be undertakne in the first trimester
  • start diagnostic monitoring with ultrasound for feto-fetal transfusion syndrome (including to identify membrane folding) from 16 weeks. Repeat monitoring fortnightly until 24 weeks
  • carry out weekly monitoring of twin and triplet pregnancies with membrane folding or other possible early signs of feto-fetal transfusion syndrome (specifically, pregnancies with intertwin membrane infolding and amniotic fluid discordance) to allow time to intervene if needed