Chapter 3. First Trimester III. Multiple Gestation

Women’s Point of Care Ultrasound

Welcome to the chapter about multiple gestation. Basics to be covered include common types of multiples, more specifically twins. The importance of early detection of the chorionicity and amnionicity of twins is presented, for predicting potential risks for later in gestation.

Objectives

By the completion of this chapter the participant will be able to:

Describe the difference between dizygotic and monozygotic twins

Recognize a multiple gestation on an image, and determine chorionicity and amnionicity

Describe the common complications encountered with multiple gestation, and the severity associated with chorionicity and amnionicity

Know the risks of preterm birth associated with twins and triplets

MULTIPLE GESTATION

The importance of identifying multiple gestation in the first trimester is to mediate the inherent risks throughout the pregnany. The information in this course will describe these risks and how to identify those multiple gestations at highest risk.

Figure 1. Importance of early detection of multiple gestation.

INCIDENCE

Incidence of twins

The global incidence of twins varies by region, but overall is around 18 per 1,000 gestations. Japan has the lowest incidence of twinning (4.3 per 1,000 pregnancies), and Western Nigeria has the highest (54 per 1,000 pregnancies). Twinning rates in different areas also differ in the type of twinning, whether monozygotic or dizygotic. ¹ ACOG (American College of Obstetrics and Gynecology) reported slightly different numbers in their 2018 practice bulletin, citing the twin birth rate was 32.6 twins per 1,000 total births in 2018.² Another statistic was that 25.5% (25.5 per 100) of women using ART (artificial reproductive technology) had multiple gestation².

Factors associated with multiple gestation

Factors associated with having a multiple gestation include:³

--Heredity

--Older age (older than 30)

--Assisted reproduction

--Ovulation stimulation/induction with medication

--In-vitro fertilization

--Past pregnancy with multiples

--Race*

*Highest rate of twinning: African descent – more likely than any other race

—Sub-Saharan Africa – highest rates of natural twinning: 6-9 per 1000 births, can be up to 18 per 1000

—A grim statistic is that one-fifth of twins in this region dies before age 5

*Lowest rate of twinning: Asian and Native Americans

Implications of twin gestation¹

Most of the risks for multiple gestation seem less to do with zygosity than with chorionicity (whether the fetuses share placental circulation).¹Sonographic assessment of chorionicity is most accurate in the first trimester.

RISKS

The most important reason for detecting multiple pregnancy during the first trimester is to increase surveillance for those pregnancies at risk for preterm delivery. It is well established that twin pregnancy is associated with a high risk of perinatal mortality and morbidity.

Multiple births account for 3% of births but 14% of infant deaths. Not suprisingly, there is a higher incidence of growth abnormalities

Risk of preterm birth

Multiple pregnancies are at increased risk of iatrogenic preterm delivery, which is significantly higher in monochorionic twin gestations. Triplets have an even greater likelihood of delivering VERY prematurely.

Table 1. Average gestational age and birth weight at delivery, comparing singleton, twin,, and triplet gestations.

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Other risks

Other risks for pregnancies with multiples include:³

--Anemia

--Malpresentation of one or both

--Intrauterine growth restriction

--Postpartum hemorrhage

--Hypertensive disorders

--Increased maternal mortality

Dichorionic twins have a fivefold increased risk compared to singleton gestations.

Monochorionic twins have a thirteen-fold increased risk for stillbirth.

Risk of neonatal death is higher than risk of stillbirth up to 34 wks

RISK OF STILLBIRTH⁴

The risk of stillbirth in twin pregnancy increases with advancing gestational age.

Dichorionic twins have a fivefold increased risk compared to singleton gestations.

Monochorionic twins have a thirteen-fold increased risk for stillbirth.

Risk of neonatal death is higher than risk of stillbirth up to 34 wks

Global stillbirth rates (2024):²

Gestation	Dichorionic	Monochorionic
35-36 wks	0.8	0.9
36-37 wks	1.5	2.8
37-38 wks	3.4	4.5
38-39 wks	10.6	9.6

*Rates per 1,000 births

Table 2. Global stillbirth rates by chorionicity of twins and gestational age.

Risks stratified by the type of twinning

Monozygotic

Monozygotic twins (derived from the same zygote) have higher incidence of structural abnormalities; and all twin gestations have increased risk of chromosomal abnormalities.

NOTE: if prenatal genetics screening is done, there is a greater risk of inaccuracy of the results.² This technology is expanding rapidly, however, and some companies are able to obtain more accurate information by separating out the twin DNA.

Monochorionic

Monochorionic twins (sharing the same placenta) have a greater risk of TTS (Twin to Twin Transfusion Syndrome)

--Risk of fetal growth restriction (FGR) is 10 times higher in twin pregnancies than in singletons

--Low birth weight of at least one of the twin occurs in up to 35% of twin gestations

--Perinatal mortality in twin pregnancies is 6 times higher than the singleton rate.

--Risk of preterm labor (birth <37 wks gestation) is approximately 60%

--Increased risk of cord and placenta-related problems: velamentous cord insertion, marginal cord insertion, and higher incidence of placenta previa spectrum

--Higher risk of pre-eclampsia²

ULTRASOUND ASSESSMENT

Because of the increased risk for twins, it is critical to learn early in the pregnancy of the presence and type of multiple gestation so complications can be mitigated when possible.^5,6

In some settings, practitioners may only be providing point-of-care ultrasound. They may or may not refer pregnant women for a routine or standard first trimester scan. This makes it even more important to confirm multiple gestation in early pregnancy by the POCUS provider.

To reiterate why early ultrasound is important, Whitworth et al: performed a Cochrane review in 2015 combining all the studies on the efficacy of routine ultrasound in first trimester⁷. This study’s purpose was to determine whether routine early pregnancy ultrasound for fetal assessment influences the diagnosis of fetal malformations, multiple pregnancies, rate of clinical interventions, and incidence of adverse fetal outcome when compared with selective use of early pregnancy ultrasound⁷. By reviewing the literature, they concluded that doing early ultrasound did increase detection of multiple pregnancy, with a moderate degree of quality evidence. Early detection of fetal abnormalities was listed as second in importance to detecting multiples.

TYPES OF TWINNING

Before scanning, one must be aware of the types of twin implantation and understand the sonographic signs of these types.

Types of twins are described by whether the fetuses are monozygotic (1 egg that splits) or dizygotic (2 separate eggs). This is a factor that determines whether twins are identical (same egg), which has implication for shared genetic concerns. This is also a factor for adequacy of placentation and fetal circulation.

This can be determined by looking for the presence or absence of a membrane separating the embryos/fetuses, as well as the configuration of the placenta. Demonstrating chorionicity is a critical assessment, because it can predict risk of potential complications related to shared circulation and or potential for cord entanglement if the fetuses are in the same amniotic sac.

Figure 2. Illustration of multiple gestation zygosity types.

Figure 3. Illustration of twinning types based on chorionicity and amnionicity.

IDENTIFYING MULTIPLE GESTATION⁸

Suspicion of Twins

The suspicion of twins should be considered when:

—Uterine size greater than dates

—Strong family history of fraternal twins

—Severe hyperemesis gravidarum

—Elevated Serum BhCG

—Pregnancy was achieved by use of reproductive technology or ovulation stimulation

Steps for imaging assessment

1. Identify number of sacs and note significant size differences

2. Identify number of fetal poles

3. Identify presence and thickness of membrane (>2mm suggests dichoronicity)

4. Identify site of membrane attaching to the placenta and look for lambda or T-sign

Early detection of multiple gestation

The earliest that a multiple gestation can be detected is around the same time that a singleton gestational sac can be identified, usually around 5 to 5.5 weeks’ menstrual age.

Because the early sac is so small, it could be overlooked or noted to be a tiny cystic area in the endometrium, rather than a sac. So it is very important to look through the entire uterus to be certain you have seen every possible area that a sac or sacs could be located.

When evaluating twins with ultrasound, we are looking for the number of gestational sacs (chorionicity) and the number of amniotic sacs (amnionicity).⁶ The amniotic sac is a sac separate from and within the chorionic sac. and is the compartment where the fetus lies. It is possible to have a combination of numbers of gestational sacs and amniotic sacs. The following illustrations and images will illustrate these differences.

Steps for imaging assessment

1. Identify number of sacs and note significant size differences

2. Identify number of fetal poles

3. Identify presence and thickness of membrane (>2mm suggests dichoronicity)

4. Identify site of membrane attaching to the placenta and look for lambda or T-sign

identifying the number of gestational sacs

The importance of sweeping through the uterus in multiple planes when assessing early pregnancy cannot be overstated. It can be easy to overlook a second or third GS. The earlier that GS sacs can be identified, the more accurate the diagnosis of type of twinning.

Images 1,2. Two views of a twin gestation, demonstrating the need for sweeping through the entire uterus.

Image 3. Transverse view of a uterus with 2 well-defined GSs and perhaps a third.

Detecting second sac vs SCH

Image 4. Image of a GS with decidua and an adjacent irregular hypoechoic area that could represent an empty resabsorbing twin sac

Image 5. Retroflexed uterus with a twin gestation. The sac closest to the cervix is empty, while the other GS shows a viable embryo.

Images 6-8 demonstrate the presence of 2 erratic gestational sacs. They are identified as gestational sacs due to being embedded into a thickened endometrium. Due to their irregularity and early gestation, it is not determined whether this will result in a viable pregnancy. Follow up ultrasound should be considered, especially if there is a discrepancy between the size of the sacs and gestational age based on LMP.

Images 6,7,8. Two gestational sacs, both empty at this gestation (5 wks). The lower image is of a CL on the right ovary.

Measuring each gestational sac separately

It is equally important to measure each gestational sac separately. Care must be taken to be sure the same sac is being measured when changing scanning angles and not mistakenly include a measurement of the other sac. It is common for the 2 sacs to be slightly different in size, so the computer will assess this in its computations.

Images 11,12. Demonstration of measuring each GS separately.

Identifying Fetal number

Once the number of gestational sacs has been determined, it is important to identify if there is a viable embryo or fetus in each sac. The condition known as “vanishing” twin occurs when one twin succumbs early in the pregnancy (eventually reabsorbed), while the other one continues as normal. In the images below, the lower right image shows a difference in GS size. In that view the fetal pole also looks markedly smaller in size. In addition to identifying sacs, also be sure that in each sac you identify either a yolk sac or fetal pole.

Images 9,10. Two gestational sacs. the smaller GS looks discrepant in size on the image to the right. However, when the transducer is angled differently, one can see the sacs are generally the same size. The fetus on the left however, had no cardiac and the fetal pole was labeled as a “vanishing twin”

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As stated at the beginning of this chapter, most of the risks for multiple gestation seem less to do more with chorionicity (whether the fetuses share placental circulation) than with zygosity

The following categories describe the relationship of chorionicity and twin type (zygosity). This categorization is important for the POCUS provider to establish.

Dichorionic diamniotic DCDA

In this type of twinning, each twin has its own chorion and amniotic sac. Most often the twins are dizygotic (derived from 2 eggs) but can occur with monozygotic (split egg).

DCDA accounts for 76% of all twin pregnancies

2 ova are independently fertilized by two sperm, leading to 2 zygotes

Sonographic findings:

2 gestational sacs with thick echogenic chorion surrounding each embryo

Thick inter-twin membrane (often >2mm)

Twin peak (lambda) sign

When there is no placental fusion, 2 separate placental sites may be seen.

DCDA carries the lowest rate of complications among twin pregnancies

Figure 4. Illustration of dichoriobic/diamniotic twins.

Lambda sign

This image shows a definite lambda sign (wing-like curve where the two separate placentas merge), as well as a thick membrane between the two gestational sacs

Image 13. Di-Di twin gestation demonstrating the Lambda sign.

In Dichorionic twinning, a wing-like curve is seen at the junction of the two placentas approximating. This is called the lambda sign. In dichorionic twins, look for the thickness of the membranes, already mentioned.

Image 14. Image of the Lambda sign.

Image 15. Di-Di twin gestation at 10 wks, demonstrating Lambda sign and thickened membrane.

Monochorionic Diamniotic

A Monochorionic Diamniotic gestation results from division of a single zygote at 4-8 days (blastocyst).

The embryos share a common gestational sac (sharing the same placenta), but each has its own separate amniotic sac and yolk sac.

This type of twinning accounts for 70-75% of monozygotic twin pregnancies, but only 30% of all twin pregnancies

The Incidence of MCDA is about 1 in 400 pregnancies

A thin inter-twin membrane is seen; thinner than with DCDA twins. It is important to see this in early gestation because it becomes more difficult to detect later in pregnancy

There is what is called the T-sign, indicating that the two amnions are equally adjacent to the placenta.

These twins are most always the same gender.

Complications of this twinning result from placental/circulation sharing, and include:

--Twin to twin transfusion syndrome (occurs in 15-30% of MCDA pregnancies)

--Demise of one twin often associated with adverse outcome to the other twin

--Placental cord insertion irregularities

Figure 5. Illustration of Monochorionic Diamniotic Twins

Monochorionic Diamniotic

Below is an image of a twin pregnancy showing 2 separate amniotic sacs but a single shared placenta (Image 17). The membrane between the fetuses is thinner than than with dichorionic diamniotic twins. The curved line where the amnion approaches the uterine wall is not wave-shaped, but T-shaped instead.

Image 16. Another Mono-Di pregnancy showing a thin membrane and a “T” configuration where it joins the placenta.

Image 17 is another example of a MONO chorionic early gestation with a T shaped attachment of the amnion.

Image 17. Mono-Di twins with a “T-sign” indicating a single placenta.

Monochorionic twin complications ⁶

Monochorionic twins pos a risk for twin-to-twin transfusion syndrome (acronym TTTS)

Other complications include

--Selective fetal growth restriction

-- Twin anemia polycythemia sequence (TAPS)

--Twin reverse arterial perfusion sequence (TRAP)

--Monoamniotic twinning

NOTE: It is not typically the role of the POCUS provider to diagnose the above conditions, but instead to be aware of these potential problems tresulting from twin zygosity and chorionicity. Some providers may provide co-management for higher risk pregnancies, or require referral/transfer of care to a high risk management provider.

Monochorionic, Monoamniotic MCMA

The last type of twinning we will look at is much rarer, and has a singnificantly increased risk. This is where there are 2 (or more) embryos or fetuses in a single amniotic sac within a single gestational sac. This type of pregnancy carries a much higher rate of potential complications, so these women need increased surveillance during the pregnancy, and if possible, a high-risk provider to care for them.

Images 18-19 below are illustrations of the sonographic view of this type of twinning.

Image 18. Image of a Mono-Mono gestation (two YS’s in the same GS.

Image 19. Mono-mono twins in the same GS and same amniotic sac.

Monochorionic Monoamniotic pregnancies account for 1-2% of all twin pregnancies, with an overall incidence of 1 in 10,000.

The findings in this type of twinning are: a single chorionic sac, single amniotic sac and generally a single YS. There is no inter-twin membrane

This results from separation of a single ovum after the trophoblast has formed, so this results in a single placenta. These are always identical and of the same gender.

MCMA is associated with a higher incidence of congenital anomalies.

In second trimester there can be cord entanglement and cord fusion, as well as cord knots and cord thrombosis. There is an increased risk of abnormal placental cord insertion. In addition, there is greater risk of twin-to-twin transfusion syndrome (TTS or TTTS), and twin embolization syndrome.

Figure 6. Illustration of Mono-mono twins.

Twin to twin transfusion

The primary risk for twins in a single gestational sac or single amniotic sac is the risk of one twin shunting blood to another due to shared circulation in the placenta. The outcomes for both twins are poor if this happens. If one fetus succumbs, there is a great likelihood that the other will also, shortly after.

Figure 7. Illustration of Twin-to-Twin Transfusion Syndrome and effects on the fetuses.

The condition known as twin-to-twin transfusion, or TTTS is a chronic and progressively worsening condition where one fetus receives less maternal blood flow through the placenta. This is a high-risk situation for the fetus who is not growing. The donor twin is also at risk for other complications such as hypovolemia, oliguria, and oligohydramnios. The recipient twin is at risk of hypervolemia, polyuria, and therefore polyhydramnios.

This is not as likely to happen in DCDA pregnancies, since each fetus typically has its own placental circulation.

Cord or limb entanglement

Figure 8. Illustration of potential for mono-mono twin entanglement.

There is a greater risk of twisting cords and entanglement with twins in the same amniotic sac. The obvious risk is accidental cord tension and compression or prolapse, both of which require rapid intervention. Another concern is for providers who only perform vaginal deliveries. With mono-mono twins, there could be limb entanglement creating a compound presentation that is not deliverable vaginally. These risks necessitate early consultation and referral of mono-mono twins for high-risk maternity care.

TRIPLET GESTATION

Triplet pregnancies carry a much greater risk than twins for complications and preterm birth. For this reason, triplet gestation should be identified early and referred for high risk management.

Triplets: Trichorionic Triamniotic

Image 20. Triplet gestation - all separate sacs and possibly trichorionic/triamniotic. Cannot rule out a “T” configuration between fetus B and C.

When evaluating for triplets, it is even more important to establish very early in the pregnancy the number of chorions or gestational sacs, and the number and location of the fetuses and amniotic sacs. This first slide shows what appears to be 3 separate chorions and 3 separate amnions.

Triplets; Dichorionic Triamniotic

Once again, it is important to carefully look through each gestational sac to be sure you do not miss a second embryo within the same sac. In this image we have a Dichorionic, and TRIamntiotic pregnancy. Image 21 (below) shows there are 2 gestations sacs, but one of them has 2 amniotic sacs each with a fetus inside.

Image 21. Dichorionic, triamniotitic triplets. Lower 2 fetuses share a GS.

Triplets: Monochorionic, Triamniotic

Image 22 is that of a monochorionic (single gestational sac) and triamniotic amnion gestation. This pregnancy is at very high risk since all 3 fetuses will share the same placenta, and there is very high risk of one or more of the fetuses succumbing.

Of course, any time a higher risk twin gestation or triplet pregnancy is diagnosed, a consultation or a referral to an obstetric specialist is indicated.

Image 22. Monochorionic, Triamniotic triplets

FETAL DEMISE OF ONE TWIN

It is not uncommon in twing pregnancy to find the demise of one twin while the other remains viable. If this occurs very early in pregnancy, often the GS of the deceased embryo will decrease in size and eventually be absorbed.

First trimester demise of one twin is not associated with adverse pregnancy outcomes.⁹

When the demise occurs late in first trimester (or later), the deceased fetus may remain in the uterus, compressed up against the uterine wall. The fetus does not decompose but is preserved in a distorted shape. This condition is known as fetus papyraceus.^1,10

Image 23. Discordant twin GS sizes. Color flow appears absent in both embryos.

The image above shows a twin gestation with markedly discordant sac sizes. The other thing to note is that there is no evidence of color flow within either of the fetal poles, indicating demise of both embryos..

The images below show a GS with either a smaller GS attached, or an area of SCH. This would require follow up with ultrasound to dermine whether growth occurs in either sac.

Image 24, 25. Early IUP with either 2 gestational sacs (smaller wtih vanishing twin), or an SCH.

In the image below (Image 25) on the far left there is a small GS without a viable fetal pole, while the other twin appears norma. The image at the right demonstrates reabsorpion of the second sac and embryo, and a normally developing fetus.

Image 25. Twin gestation with embryonic demise of twin b, and subsequent normal growth of twin A (image on the right).

Both twins in the below image (26) are seen and appear normal. The image on the right was at a different angle of the probe dissecting the uterus, and if that was the only image obtained by the POCUS exam, the conclusion could be that this was a failed twin pregnancy. This highlights once again the need to view the uterus in all planes to detect things that may not be see in one plane.

Image 26. Twins at 8w0d. Two views

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The images below demonstrate the progression of a demise of one twin at 8 wks gestation. The sac for the second twin remains but there is no fetal pole or YS noted. The sac is also irregular in shape and smaller than the sac with the viable pregnancy. Later, at 10 weels. the sac is seen as reduced in size and collapsing toward the uterine wall. This sac should eventually collapse altogether.

Images 27, 28. Top image is one viable fetus on the left and an empty GS on the right. Bottom image shows the same pregnancy at 10 wks with the sac of the demised fetus shrinking.

DOCUMENTATION:

Labeling of the twin fetuses is an important process for the POCUS provider.⁵ Considered a good practice point by ISUOG, this identification should follow a reliable and consistent strategy and should be recorded accurately in the woman’s medical record. It should begin with identifying the gestational sacs in either a longitudinal or transverse orientation.

First trimester documentation should include: dating, labeling, chorionicity, and genetic screening results (where available).⁵

Image 29. Images of normal twin gestation at 8.5 wks

Once the fetuses have been identified, it is important to label the fetus closest to the cervix as Twin A, and the other Twin B. If the twins are lateral to each other, they can be labeled as left or right. Most ultrasound machines will have a preset when entering the patient information to identify how many gestations will be measured. Once you select that number, the computer will always start with Twin A (that you have identified). Then you will measure all the parameters associated with Twin A. Once that is completed, then the computer will allow you to switch to Twin B to measure those parameters as separate from the Twin A measurements. It is important to measure each twin one at a time, correctly identifying the structures of that twin as separate from the other. Often it is easier to distinguish between the two if they are in different positions, or if the amnion is clearly seen between the two.

SUMMARY

There is increased morbidity and mortality associated with multiple pregnancy
Early diagnosis of the number of sacs and implantation are important to establish chorionicity and zygosity
Zygosity and chorionicity impact the incidence of pregnancy complications, with dichorionic/dizygotic pregnancies at the lower end of the risk spectrum and monochorionic/monoamniotic pregnancies at the higher end.
POCUS providers should seek consultation and referral for management of high risk twin pregnancies.

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Women's Point-of-Care Ultrasound: First Trimester

First Trimester III Multiple Gestation