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 a twin or triplet pregnancy a first trimester ultrasound scan to estimate gestational age and determine chorionicity and amnionicity (ideally, these should all be performed at the same scan
- chorionicity
- number of chorionic (outer) membranes that surround babies in a multiple pregnancy. If there is only 1 membrane, the pregnancy is described as monochorionic; if there are 2, the pregnancy is dichorionic; and if there are 3, the pregnancy is trichorionic. Monochorionic twin pregnancies and monochorionic or dichorionic triplet pregnancies carry higher risks because babies share a placenta
- amniocity
- number of amnions (inner membranes) that surround babies in a multiple pregnancy. Pregnancies with 1 amnion (so that all babies share an amniotic sac) are described as monoamniotic; pregnancies with 2 amnions are diamniotic; and pregnancies with 3 amnions are triamniotic
- determine chorionicity and amnionicity at the time of detecting a twin or triplet pregnancy by ultrasound using:
- the number of placental masses
- the presence of amniotic membrane(s) and membrane thickness
- the lambda or T-sign.
- 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
- if a woman with a twin or triplet pregnancy presents after 14+0 weeks, determine chorionicity and amnionicity at the earliest opportunity by ultrasound using all of the following:
- the number of placental masses
- the presence of amniotic membrane(s) and membrane thickness
- the lambda or T-sign
- discordant fetal sex
- If it is not possible to determine chorionicity or amnionicity by ultrasound at the time of detecting the twin or triplet pregnancy, seek a second opinion from a senior sonographer or refer the woman to a healthcare professional who is competent in determining chorionicity and amnionicity by ultrasound scan as soon as possible
- if it is difficult to determine chorionicity, even after referral (for example, because the woman has booked late in pregnancy), manage the pregnancy as a monochorionic pregnancy until proven otherwise
- 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
- at each ultrasound scan from 24 weeks, offer women with a dichorionic twin or trichorionic triplet pregnancy diagnostic monitoring for fetal weight discordance using 2 or more biometric parameters and amniotic fluid levels. To assess amniotic fluid levels, measure the deepest vertical pocket (DVP) on either side of the amniotic membraneestimate fetal weight discordance using two or more biometric parameters
- continue monitoring for fetal weight discordance at intervals that do not exceed:
- 28 days for women with a dichorionic twin pregnancy
- 14 days for women with a trichorionic triplet pregnancy
- calculate and document estimated fetal weight (EFW) discordance for dichorionic twins using the formula below:
- ([EFW larger fetus − EFW smaller fetus] ÷ EFW larger fetus) × 100
- calculate and document EFW discordance for trichorionic triplets using the formula below:
- ([EFW largest fetus − EFW smallest fetus] ÷ EFW largest fetus) × 100 and
- ([EFW largest fetus − EFW middle fetus] ÷ EFW largest fetus) × 100
- inrease diagnostic monitoring in the second and third trimesters to at least weekly, and include Doppler assessment of the umbilical artery flow for each baby, if:
- there is an EFW discordance of 20% or more and/or
- the EFW of any of the babies is below the 10th centile for gestational age
- refer women with a dichorionic twin or trichorionic triplet pregnancy to a tertiary level fetal medicine centre if there is an EFW discordance of 25% or more and the EFW of any of the babies is below the 10th centile for gestational age because this is a clinically important indicator of selective fetal growth restriction
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
- offer diagnostic monitoring for feto-fetal transfusion syndrome to women with a monochorionic twin or triplet pregnancy. Monitor with ultrasound every 14 days from 16 weeks until birth
- use ultrasound assessment, with a visible amniotic membrane within the measurement image, to monitor for feto-fetal transfusion syndrome. Measure the DVP depths of amniotic fluid on either side of the amniotic membrane
- increase the frequency of diagnostic monitoring for feto-fetal transfusion syndrome in the woman's second and third trimester to at least weekly if there are concerns about differences between the babies' amniotic fluid level (a difference in DVP depth of 4 cm or more). Include Doppler assessment of the umbilical artery flow for each baby
- refer the woman to a tertiary level fetal medicine centre if feto-fetal transfusion syndrome is diagnosed, based on the following:
- the amniotic sac of 1 baby has a DVP depth of less than 2 cm and
- the amniotic sac of another baby has a DVP depth of:
- over 8 cm before 20+0 weeks of pregnancy or
- over 10 cm from 20+0 weeks
- refer the woman to her named specialist obstetrician for multiple pregnancy in her second or third trimester for further assessment and monitoring if:
- the amniotic sac of 1 baby has a DVP depth in the normal range and
- the amniotic sac of another baby has a DVP depth of:
- less than 2 cm or
- 8 cm or more
For full details see NICE (April 2024). Twin and triplet pregnancy
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