Early pregnancy ultrasound plays an important role in confirmation of pregnancy, establishing viability and location of the pregnancy, confirming fetal number and type of placentation in multiple pregnancy and facilitating early diagnosis of life-threatening abnormalities such as ectopic or molar pregnancy. In this chapter, we will provide the reader with key information on the value of early pregnancy ultrasound.

Abstract: Early pregnancy ultrasound plays an important role in confirmation of pregnancy,  establishing viability and location of the pregnancy, confirming fetal number and type of  placentation in multiple pregnancy and facilitating early diagnosis of life-threatening abnormalities such as ectopic  or molar pregnancy. In this chapter, we will provide the reader with key information on the value of early pregnancy ultrasound. We also briefly cover embryologic milestones to help better understand ultrasound findings and their significance. This chapter focuses mainly on normal findings up to the 11th week of gestation. For detailed information on the ultrasound examination in the late first trimester, or abnormal findings such as ectopic pregnancy, please refer to the corresponding VISUOG chapter.

Key words: early pregnancy, ultrasound, first trimester, normal, embryo, fetus, transvaginal ultrasound, pregnancy viability

Author: Mauro H. Schenone, MD1

  1. University of Tennessee Health Science Center - Regional One Health Department of Obstetrics and Gynecology
     

Reviewer: Karen Fung-Kee-Fung

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Introduction

Advances in ultrasound technology, especially transvaginal ultrasound systems, have facilitated early identification of pregnancy and its complications. Along with early diagnosis of pregnancy and its viability, early pregnancy ultrasound enables identification of the  number of fetuses/embryos, establishes chorionicity and amnionicity, and detects life threatening conditions such as ectopic and molar pregnancy. 1 Towards the end of the first trimester, ultrasound examinations also offer an opportunity to screen for genetic and structural  abnormalities by identification of soft aneuploidy markers ( i.e. by measuring the nuchal translucency thickness) and early review of fetal anatomy, enabling early diagnosis of many structural anomalies. It is acknowledged that many gross malformations may not be apparent until later in pregnancy or may not be detected in early pregnancy even with appropriate equipment and in experienced hands. 2 

This chapter focuses mainly on normal findings up to the 11th week of gestation. For detailed information on the ultrasound examination in the late first trimester, or abnormal findings such as ectopic pregnancy, please refer to the corresponding VISUOG chapter. For more details and guidelines on performance of first trimester ultrasound we suggest consulting ISUOG guidelines highlighting key components of the first trimester fetal ultrasound scan. 2


For purposes of definition, in this chapter, we will use menstrual age (pregnancy dated from first day of last menstrual period) as the means to describe the gestational age of the pregnancy, unless otherwise specified as conceptional age (or age in weeks and days from actual date of conception).

Objectives of first trimester ultrasound

•    Confirm pregnancy viability 
•    Establish gestation age (most reliable early in pregnancy) 2
•    Establish the number of fetuses
•    Define chorionicity and amnionicity (most reliable early in pregnancy) in multiple gestation 2
•    Screen for anomalies detectable at this gestational age (cautioning the patient that many malformations may develop later in gestation and other can be missed due to early gestational age)
•    Aneuploidy screening with nuchal translucency (11-14 weeks) 

Gestational Sac

•    The gestational sac becomes visible to transvaginal ultrasound by 4-4.5 weeks of gestation. At this gestational age it appears as a small hypoechoic/anechoic cystic structure contained within the decidualized endometrium and off the midline. Such appearance has been denominated as the “intradecidual sac sign”. 3
•    The gestational sac normally appears rounded, smooth-walled and regular during early pregnancy. Later in gestation, it becomes oval but maintains smooth and regular walls.
•    In the intrauterine pregnancy the decidua parietalis and the decidua capsularis present as concentric rings to form what has been described as the double sac sign. 4 
•    The mean gestational sac diameter is obtained by averaging the measurements of the three orthogonal planes of the fluid-filled space within the gestational sac (inner to inner) 2. Normative values have been published previously 5-9.
 

Yolk Sac

•    The yolk sac is visible as early as 5-5.5 weeks using transvaginal ultrasound. 3
•    Its normal appearance includes the following elements: rounded, smooth and regular contour with echogenic border and anechoic center. It is usually not visible after first trimester. 3 Yolk sac largest diameter is measured placing calipers “inner to inner”. 

Embryo

•    The embryo is first detectable in transvaginal ultrasound when 1-2 mm in length2 and between 5 weeks and 5 days and 6 weeks and 3 days.10 A distinctly identifiable embryo with cardiac activity is notable at 6-6.5 weeks. 3
•    The cardiac activity has been detected in embryos as early as 37 days11  The embryonic heart rate increases with gestational age until around 63 postmenstrual days or CRL of 22 mm and starts decreasing thereafter. 12 Normal values (mean  2 standard deviations) previously reported include 128.6  19.14; 149.2  30.4; 169  20.78; 174.6  24.24; 165.9  11.88; 161.8  16.72  at 6, 7, 8, 9 ,10 and 11 weeks of gestation respectively. 13
•    Studies have shown that embryos have near identical growth velocity. 14
•    The embryo is first visible as an echogenic area on one side of the yolk side between 5 weeks and 5 days and 6 weeks and 3 days . The embryo this early in pregnancy has a laminar or linear appearance and as the gestational age advances it elongates and later starts curling/folding. The yolk sac is closely related to the ventral side of the embryo and on the opposite side of the amnion is still limited to the dorsal part of the embryo (double bleb sign).1,10,15
•    At 7 weeks of gestation the following are noticeable: two lines in the dorsal and coronal plane demarcating the developing spine, limb buds.10 The cephalic and caudal ends of the embryo are generally indistinguishable by ultrasound until the CRL reaches 12 mm and gestational age is 53 days (7.5 weeks) when the rhombencephalon becomes apparent.2 The physiologic gut herniation may be detected .
•    At 8 weeks is when the rhombencephalon is best and most distinctly seen and shows as a small cystic structure (3-4 mm) in the posterior aspect of the cranium . The rhombencephalon becomes the 4th ventricle by week 11 of gestation.16 Embryonic movement has been described at 8 weeks and 2 days.10

At 9 weeks the developing spine is better visualized and can be followed from uppermost part down to the sacrum. The division between ventricles becomes visible and the choroid plexi are notable. In the sagittal plain two anechoic areas are seen the ventricle in the forebrain and another one posteriorly representing the hind brain. 10 
•    It is only about 10 weeks of gestation when the fetus starts to resemble more distinctly a human being .1 

Physiologic gut herniation is visualized through transvaginal ultrasound starting at 7 weeks of gestation and ends at 10-12 weeks of gestation. Timor-Tritsch et al. showed 100 % resolution by menstrual week.12 17 

•    The embryo is measured at its longest axis. From the most cephalad point of the embryo to the most caudal point of the rump; crown-rump length (CRL). Later in first trimester it is recommended to secure a “neutral” position of the neck (no flexion or extension). CRL is the most accurate way of dating a pregnancy with ultrasound.
 

Amniotic sac

The amniotic sac is distinct from the chorion during the first trimester. Normative values have been previously reported. 7,18 

Chorionic cavity

Chorionic cavity is the space between the amnion and the chorion. Obliterates when the amnion and chorion fuse (normally not later than 16 weeks of gestation). 3

Decidua and early placenta

Decidua and early placenta: The decidua is a product of hormonally induced changes of the endometrium. It is comprised of the following component decidual basalis, capsularis and parietalis. Although the conventional wisdom is that placental location cannot be predicted or ascertained before 8 weeks of gestation, a recent study challenges such statement by using the embryonal and yolk sac position at 5-6 weeks of gestation.19

Early umbilical cord and its embryonic origin

The umbilical cord is formed by allantoic duct and vessels, connecting stalk and vitelline duct. It starts very short and it lengthens and coils as gestational age advances.

Multiple gestation

•    Number of fetuses: Early pregnancy ultrasound and sweeping of the ultrasound probe side to side and up and down allows for visualization of the entire uterine cavity and fetal count.
          
•    Chorionicity: The findings used to determine chorionicity include: shape of the intertwin membrane junction with the placenta, thickness of the intertwin membrane (thicker in dichorionic gestations), and fetal gender (discordant gender makes the diagnosis of dichorionic situation).

•    Amnionicity: The intertwin membrane visualization guides the diagnosis of amnionicity. When no intertwin membrane is visualized after 8.5 weeks, a monoamniotic gestation is suspected. Prior to this time the amnion is not a good specular reflector and a false diagnosis of monoamniotic twins can be made  Evaluation of umbilical cords and their placenta insertion further helps in the diagnosis of a monoamniotic gestation (e.g., cord entanglement).

Uterus

Normal appearance in pregnancy, smooth external contour and homogeneous echogenicity of the myometrial layer along with a more echogenic endometrial layer. Scanning side to side and cephalad to caudad will secure that we have inspected the entire uterus during our examination. It is important to evaluate the contour of the uterus in order to help ascertain and rule out Mullerian abnormalities, leiomyomas or other neoplasias (see Video 1. Imaging the entire uterus and uterine cavity).

Ovaries

•   Normal appearance in pregnancy: the ovaries have a distinct echogenicity that allows for its identification from the rest of the adnexal tissues. Cystic structures can normally be seen within the ovaries and correspond to follicles. The corpus luteum can be seen as a mass in the ovary with particular echogenicity and surrounded by vascularity (“ring of fire”). Nevertheless, it can also be cystic, and at times contain blood or a clot (hemorrhagic corpus luteum). 

Early pregnancy viability can be determined using ultrasound. 


•    Findings diagnostic of pregnancy failure include the following (20):

-Crown–rump length of ≥7 mm and no embryonic heart activity detected 
-Mean sac diameter of ≥25 mm without a visible embryo
-Absence of embryo with heartbeat ≥2 wk after an ultrasound examination that showed a gestational sac without a yolk sac
-Absence of embryo with heartbeat ≥11 days after an ultrasound examination that showed a gestational sac with a yolk sac

•    Findings suspicious for, but not diagnostic of, pregnancy failure include the following (20):

-Crown–rump length of <7 mm and no embryonic heart activity detected 
-Mean sac diameter of 16–24 mm and without a visible embryo
-Absence of embryo with heartbeat 7–13 days after an ultrasound examination that showed a gestational sac without a yolk sac
-Absence of embryo with heartbeat 7–10 days after an ultrasound examination that showed a gestational sac with a yolk sac
-Absence of embryo ≥6 wk after last menstrual period
-Empty amnion (amnion seen adjacent to yolk sac, with no visible embryo)
-Enlarged yolk sac (>7 mm)
-Small gestational sac in relation to the size of the embryo (<5 mm difference between mean sac diameter and crown–rump length)

Other incidental findings in early pregnancy ultrasound

•    Subchorionic hematoma : the shape is variable and generally oval or lenticular. Subchorionic hematomas start as echogenic comparable to placental tissue and with time they evolve to hypoechoic. Most subchorionic hematomas in early pregnancy ultrasound resolve with time and result in delivery of a normal neonate. Retroplacental location, involvement of more than 50% of the sac circumference, are among the factors that predict poor outcome. 21
•    Large yolk sac has been associated with adverse pregnancy outcome, particularly genetic abnormalities. 22
•    Chorionic bump:  A chorionic bump in early pregnancy has been associated with an increased risk of spontaneous abortion. 23

References

Special thanks to Laura Detti, MD; Shabnam Bobdiwala, MD; Jennifer Carelle, MSHS, RDMS, RVT, RT(R); Michelle Walker BHS, RDMS, RDCS for their contribution to this chapter. 

1.   Blaas HG. The examination of the embryo and early fetus: how and by whom? Ultrasound Obstet Gynecol. 1999;14(3):153-158.
2.  Salomon LJ, Alfirevic Z, Bilardo CM, et al. ISUOG practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2013;41(1):102-113.
3.    Woodward PJ. Diagnostic imaging. Obstetrics. 2nd ed. Salt Lake City, Utah: Amirsys; 2011.
4.    Bradley WG, Fiske CE, Filly RA. The double sac sign of early intrauterine pregnancy: use in exclusion of ectopic pregnancy. Radiology. 1982;143(1):223-226.
5.    Oh JS, Wright G, Coulam CB. Gestational sac diameter in very early pregnancy as a predictor of fetal outcome. Ultrasound Obstet Gynecol. 2002;20(3):267-269.
6.    Robinson HP. "Gestation sac" volumes as determined by sonar in the first trimester of pregnancy. Br J Obstet Gynaecol. 1975;82(2):100-107.
7.    Grisolia G, Milano K, Pilu G, et al. Biometry of early pregnancy with transvaginal sonography. Ultrasound Obstet Gynecol. 1993;3(6):403-411.
8.    Abdallah Y, Daemen A, Guha S, et al. Gestational sac and embryonic growth are not useful as criteria to define miscarriage: a multicenter observational study. Ultrasound Obstet Gynecol. 2011;38(5):503-509.
9.    Bottomley C, Daemen A, Mukri F, et al. Assessing first trimester growth: the influence of ethnic background and maternal age. Hum Reprod. 2009;24(2):284-290.
10.    Timor-Tritsch IE, Farine D, Rosen MG. A close look at early embryonic development with the high-frequency transvaginal transducer. Am J Obstet Gynecol. 1988;159(3):676-681.
11.    Jurkovic D, Gruboeck K, Campbell S. Ultrasound features of normal early pregnancy development. Curr Opin Obstet Gynecol. 1995;7(6):493-504.
12.    Wisser J, Dirschedl P. Embryonic heart rate in dated human embryos. Early Hum Dev. 1994;37(2):107-115.
13.    Robinson HP, Shaw-Dunn J. Fetal heart rates as determined by sonar in early pregnancy. J Obstet Gynaecol Br Commonw. 1973;80(9):805-809.
14.    Blaas HG, Eik-Nes SH, Bremnes JB. The growth of the human embryo. A longitudinal biometric assessment from 7 to 12 weeks of gestation. Ultrasound Obstet Gynecol. 1998;12(5):346-354.
15.    Timor-Tritsch IE, Peisner DB, Raju S. Sonoembryology: an organ-oriented approach using a high-frequency vaginal probe. J Clin Ultrasound. 1990;18(4):286-298.
16.    Cyr DR, Mack LA, Nyberg DA, Shepard TH, Shuman WP. Fetal rhombencephalon: normal US findings. Radiology. 1988;166(3):691-692.
17.    Timor-Tritsch IE, Warren WB, Peisner DB, Pirrone E. First-trimester midgut herniation: a high-frequency transvaginal sonographic study. Am J Obstet Gynecol. 1989;161(3):831-833.
18.    Zimmer EZ, Chao CR, Santos R. Amniotic sac, fetal heart area, fetal curvature, and other morphometrics using first trimester vaginal ultrasonography and color Doppler imaging. J Ultrasound Med. 1994;13(9):685-690.
19.    Detti L, Gordon JC, Christiansen ME, et al. Diagnosis of Placental Position by Early First-Trimester Ultrasound: A Pilot Study. Reprod Sci. 2019:1933719119831778.
20.    Doubilet PM, Benson CB, Bourne T, et al. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med. 2013;369(15):1443-1451.

This article should be cited as: Schenone M: Normal Early Pregnancy, Visual Encyclopedia of Ultrasound in Obstetrics and Gynaecology, www.isuog.org, January 2022.


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