Multiple pregnancy — chorionicity, surveillance and timing of birth — structured SAQ (15 marks)

(a) Chorionicity and amnionicity (3 marks)

One mark per point with the determining sign. ^[1][6]

• Monochorionic diamniotic (MCDA): single placental mass with a T-sign (the inter-twin membrane inserts flat onto the placenta with no triangular projection of placental tissue).
• Amnionicity is diamniotic because there is an inter-twin membrane (two amniotic sacs); a monoamniotic pregnancy would have no separating membrane.
• Lambda (delta) sign would indicate dichorionic — its absence with a single placenta confirms monochorionic. Chorionicity is most reliably determined before 14 weeks and drives all subsequent management.

(b) Surveillance schedule and complications (4 marks)

One mark for the schedule, three for the complications being screened. ^[1][5]

• 2-weekly growth scans from 16 weeks with amniotic fluid (deepest vertical pool) and Doppler each visit — this is more intensive than for dichorionic twins (4-weekly from 20 weeks) because of the shared circulation.
• Twin-twin transfusion syndrome (TTTS) — polyhydramnios in the recipient (DVP above 8 cm at 20 weeks, above 10 cm after) with oligohydramnios in the donor (DVP below 2 cm); Quintero staging; refer for fetoscopic laser ablation for stage II to IV at 16 to 26 weeks.^[5]
• Twin anaemia polycythaemia sequence (TAPS) — chronic fetofetal transfusion without polyhydramnios; diagnosed by discordant MCA-PSV (above 1.5 MoM in one, below 1.0 MoM in the other).^[5]
• Selective fetal growth restriction — discordant growth (over 20% using the larger twin as denominator) with abnormal umbilical artery Doppler in the smaller twin, from unequal placental sharing.

(c) Timing and mode of birth with evidence (4 marks)

One mark per element with the trial. ^[1][2][3]

• Timing: planned birth from 36 weeks, no later than 37 weeks for uncomplicated MCDA twins — the stillbirth risk in monochorionic twins rises after 36 weeks even when surveillance has been reassuring.
• Mode of birth: planned vaginal birth is reasonable if twin 1 is cephalic — the Twin Birth Study (Barrett, NEJM 2013) randomised 2804 women with twins and twin 1 cephalic to planned caesarean versus planned vaginal and found NO difference in neonatal mortality or morbidity.^[2]
• Caesarean is planned if twin 1 is non-cephalic or for standard obstetric indications.^[2]
• For dichorionic twins (for contrast) planned birth is from 37 weeks, no later than 38, supported by the Twins Timing of Birth Trial (Dodd, BJOG 2012).^[3]

(d) Progesterone for preterm birth prevention (2 marks)

One mark for the answer, one for the evidence. ^[4]

• No — progestogens do not prevent spontaneous preterm birth in multiple pregnancy, and should not be prescribed for this indication.
• The Cochrane overview (Dodd 2019) found no evidence of benefit in women with a multiple pregnancy, in contrast to the established benefit in singleton pregnancies with a short cervix or previous preterm birth.

(e) Maternal risks and antenatal approach (2 marks)

One mark for the risks, one for the approach. ^[1]

• Maternal risks: hypertensive disease (2 to 3 times singleton risk), gestational diabetes, anaemia, antepartum haemorrhage, operative delivery, postpartum haemorrhage (overdistended uterus).
• Antenatal approach: blood pressure and urinalysis at every visit, low-dose aspirin for pre-eclampsia prophylaxis (she has multiple pregnancy as a risk factor), full blood count for anaemia, oral glucose tolerance test at 24 to 28 weeks, and counselling on postpartum haemorrhage risk.