Twins

Question 1

What is the optimal time to determine chorionicity in a twin pregnancy?

Answer 1

10-14 weeks

Question 2

What cut-offs are used to diagnose discordant growth of twins?

Answer 2

difference in EFW > 20%

difference in AC > 20 mm

Question 3

At what gestational age should twins be delivered:

• Mono/mono
• Mono/di
• Dichorionic

Answer 3

M/M: 32-33 weeks
M/D: 36-37 weeks
DC: 37-38 weeks

Question 4

List three reasons why perinatal morbidity & mortality is higher with twin pregnancies compared with singletons.

Answer 4

More common:

• PTB, PPROM
• IUGR, HDP
• Congenital anomalies
• Twin-specific problems such as TTTS
• CP

Question 5

Distinguish between monozygotic and monochorionic twins.

Answer 5

Monozygotic: “identical twins,” egg + sperm = single zygote which splits to produce twins
Monochorionic: obligately monozygotic, single shared placenta

Question 6

List four items on the differential diagnosis for increased SFH.

Answer 6

Multiple pregnancy
Macrosomia
Polyhydramnios
Fibroids
Molar pregnancy
Inaccurate dating
Adnexal or abdominal mass

Question 7

List five factors which increase the rate of twinning, and specify whether these factors increase rates of monozygotic twinning, dizygotic twinning, or both.

Answer 7

Black race
Increased maternal age
Parity
Family history of twins (maternal > paternal)
Nutrition
Elevated levels of pituitary gonadotropins
Infertility, ART use

(Only use of ART increases the rate of monozygotic twinning - rate is otherwise constant at 1 in 250 births)

Question 8

Distinguish between the placental anastomoses responsible for TTTS & TRAP.

Answer 8

TTTS - arteriovenous anastomoses

TRAP - arterioarteriolar anastomoses

Question 9

Describe the Quintero stages of TTTS.

Answer 9

1 - poly/oli
2 - poly/oli plus absent bladder in donor twin
3 - poly/oli, absent bladder, plus abnormal Dopplers
4 - ascites or hydrops in either twin
5 - demise of either twin

Question 10

Which of the following therapies has evidence for reducing preterm delivery in twin pregnancies:

Bedrest
Prophylactic tocolysis
IM progesterone
PV progesterone
Cerclage
Pessary

Answer 10

Pessary only

Question 11

In a twin dating ultrasound, which CRL should be used to establish the due date, and why?

Answer 11

Larger twin - no good evidence but this is the conservative option in that it reduces the chance of missing IUGR later on

Question 12

List three ways twin chorionicity & amnionicity can be determined prior to and after 10 weeks’ gestation.

Answer 12

Before 10 weeks: number of gestational sacs (each forms its own chorion), number of amniotic sacs, number of yolk sacs (each is associated with a single amnion)

After 10 weeks:fetal genitalia, placental number, chorionic peak sign (lambda or T sign), inter-twin membrane thickness (2 mm threshold for chorionicity), cord or limb entanglement

Question 13

What specific kind of fetal anomalies are most common in twins & why?

Answer 13

Midline structural defects - resulting from the twinning process in monozygotic twins
(Frequency of congenital anomalies not thought to be increased in dizygotic twins)

Question 14

What options exist for prenatal screening in twin pregnancies?

Answer 14

Maternal age
NT (for each twin in dichorionic pregnancies, average of both in monochorionic pregnancies)
Serum screening: beta-hCG, PAPP-A (few studies, medians must be adjusted for twin pregnancies but distribution in twin pregnancies unclear, does not appear superior to NT + MA)
Second trimester serum screen (better than age alone, but FPR approx 10% therefore results in unnecessary invasive testing)
Second trimester US (soft markers)