- First of all, an accurate understanding of first trimester ultrasound findings requires application of appropriate terminology (Table 1).
- First trimester ultrasound is important to confirm viability, stablish gestational age accurately, determine number of embryos and, in case of multiple pregnancy, assess chorionicity and amnionicity.
- In this article the term “age” is expressed as menstrual or gestational age.
- On routine transvaginal ultrasound the sequence of events in the early pregnancy follows a clearly predictable pattern. Lets take a look at…
COMMONLY USED TERMINOLOGY
- LAST MENSTRUAL PERIOD: It refers to the most recent menstrual period. Pregnancy is dated in weeks starting from the first day of LMP.
- GESTATIONAL AGE: Also known as menstrual age. It's the term given in weeks and days to describe the human development timed from the first day of the LMP. When LMP is unknown or encertain the gestational age is given by fetal biometry.
- FIRST TRIMESTER: Pregnancy stage from the confirmation of embryo viability up to 13+6 weeks.
- VIABILITY: Refers to a pregnancy which potentially results in a liveborn baby (e. g. an intrauterine gestational sac with an embryo demonstrating cardiac activity).
- EMBRYO: The early stage of intrauterine human development until 10 gestational weeks.
- FETUS: The embryo after 10 gestational weeks (reflects the fact that after this period the organogenesis is essentially complete).
Table 1. Common terms used in first trimester ultrasound
Fig. 1 - Gestacional age x Embryonic Age. The gestational age is the time expressed in weeks measured from the first day of the last menstrual period to the current date. On the other hand, embryonic age is measured from the first day of conception to current date. Therefore, as conception only occurs after ovulation (about 14 days after menstruation), we should say that the embryonic age is 2 weeks less than gestational age.
The earliest sonographic finding of an intrauterine pregnancy (IUP) is the visualization of the gestational sac at approximately 5 weeks after LMP (3 weeks after conception), appearing as a 2-3 mm rounded cystic-fluid structure within the upper third of the thickened decidualized endometrium (Fig. 2). Although some signs (e.g. intradecidual sign) are reported as highly suggestive of an IUP, they are absent in at least 35% of the gestational sacs. Therefore, a round or oval intrauterine fluid collection in a woman with positive b-hcg should be treated as a gestational sac until proven otherwise.
Mean sac diameter (MSD): it is the first sonographic measurement used to estimate the gestational age before an embryo can be depicted within the GS. Its growth rate is about 1 mm/day in early pregnancy but is quite variable. The MSD is obtained as the average of the three gestational sac orthogonal diameters with the calipers placed within its inner walls (Fig. 3). On transvaginal scan, a MSD of 6 mm should contain an yolk sac and a MSD of 10 mm should contain an embryo.
Fig.2 – Intradecidual sign in an early intrauterine pregnancy. Sagittal view of the uterus depicts the gestational sac as an intrauterine fluid collection (arrow) lying eccentrically within the upper third of the decidua posterior to an echogenic line (arrowheads) representing the undisturbed collapsed uterine cavity.
Fig. 3– The MSD is the average taken by gestational sac measurements: (height + length + width)/3. A: longitudinal scan; B: transverse scan. 1, height; 2, length; B: 3, width.
The gestational sac represents the chorionic cavity and its echogenic rim represents the implanting chorionic villi and associated decidual tissue.
It is the first intragestational sac structure to be depicted at ultrasound and that confirms an intrauterine pregnancy (100% PPV). The yolk sac (YS) is the primary source of exchange between the embryo and the mother and has nutritive, endocrine, metabolic, immunologic, secretory, excretory and hemopoietic functions. YS can be visualized at about 5.5 weeks of gestational age as a 3-5 mm spherical-shaped structure with a sonolucent center and an echogenic periphery usually located eccentrically within the GS (Fig. 4). In an early stage the yolk sac may appear as two parallel lines rather than a round structure (Fig. 5).
YS diameter should be taken from the distance between its inner walls and grows to a maximal size of 6 mm by 10 weeks gradually migrating to the periphery of the chorionic cavity.
Fig. 4 – Note the yolk sac (arrow) as a spherical hyperechoic ring eccentrically situated within the gestational sac (*).
Fig. 5 – The yolk sac (arrow) seen as two parallel lines – representing the front edge and the backside wall – within the gestational sac (*).
The embryo is first depicted at about 6 weeks of gestational age as a 1-2 mm focal thickening at the periphery of the yolk sac – the embryonic pole – an appearance referred to as the “diamond ring” sign (Fig. 6). The embryo length is the most accurate measurement to estimate gestational age through the first 12 weeks and is taken from the head (crown) to the buttocks (rump), therefore the term crown-rump length (CRL). By 6-7 weeks the embryo is up to 10 mm in length and is still fairly featureless (Fig. 7). Usually the embryo size increases by around 1 mm/day. Cephalic and caudal ends are indistinguishable until 53 days (around 12 mm).
Fig. 6 – Transvaginal ultrasound showing the “diamond ring” sign within the gestational sac. The yolk sac (arrowhead) represents the “ring” and the embryo (arrow) represents the “diamond” in this earliest sign of embryonic development.
Fig. 7 – In this very early pregnancy it is not possible to distinguish between the cephalic and caudal ends of the embryo, hence the greatest length measurement is taken to accurately estimate the gestational age.
Cardiac pulsation in the two paired endocardial heart tubes begins at approximately the 6th week of gestation. Therefore, cardiac activity should be observed soon as the embryo reaches a length of 1-2 mm. Embryonic heart rate increases with gestation from a mean of about 110 bpm and a lower limit of 100 bpm at 6 week to a maximum of about 175 bpm at 9 weeks with decreasing thereafter. The early increase in HR coincides with the morphological development of the heart, and the subsequent decrease may be the result of functional maturation of the parasympathetic system.
Fig. 8 - Fetal heart rate determinated by M-mode. The waveform represents the movement of the beating heart. Using the caliper function we calculate the fetal heart rate by measuring from peak-to-peak (arrows) of two subsequent waves.
Table 2. Relationship between gestational age (GA) and embryonic heart rate (HR).*w=weeks;d=days
By 7.0 weeks the amnio becomes visible as a thin hyperechoic membrane surrounding the embryo. Over time the amniotic cavity (Fig. 9) gradually expands as the fetus produces urine, promoting the fusion between the amnion and chorionic membrane and the progressive desappearing of the extracoelomic cavity.
Fig. 9- The embryo (e) is surrounded by a thin membrane, the amnion (arrowheads). Fluid (*) is present within the amniotic cavity, separating the amnion from the embryo.
By 8 weeks the embryo head can be depicted as a separate structure from the body and the limb buds are visible. From 8 to 10 weeks an anechoic round structure can be seen within the fetal head (FIg. 10). It represents the developing rhombencephalon, also known as hindbrain, the brain structure comprising the medulla, pons and cerebellum.
Fig. 10- The rhombencephalon (arrowhead) is depicted as an intracranial cystic space about 8 weeks and corresponding to the developing hindbrain.
PHYSIOLOGIC BOWEL HERNIATION
About 10 to 13 weeks, an echogenic structure should be seen protruding through the anterior abdominal wall (Fig. 11). This ultrasound finding corresponds to a physiologic bowel herniation, where intestinal structures temporarily extends into the base of the umbilical cord.
Fig. 11- Physiologic bowel herniation. Sagittal view of a 10-week fetus shows a soft tissue mass (arrow) protruding from the anterior abdominal wall, corresponding to a stage in the normal embryologic development of the gastrintestinal tract in which the intestine temporarily extends into the base of the umbilical cord.
1. Doubilet, Peter M. "Ultrasound evaluation of the first trimester." Radiologic Clinics 52.6 (2014): 1191-1199.
2. Bottomley, Cecilia, and Tom Bourne. "Dating and growth in the first trimester." Best practice & research Clinical obstetrics & gynaecology 23.4 (2009): 439-452.
3. Salomon LJ, Alfirevic Z, Bilardo CM, Chalouhi GE, Ghi T, Kagan KO, Lau TK, Papageorghiou AT, Raine‐Fenning NJ, Stirnemann J, Suresh S, Tabor A, Timor‐Tritsch IE, Toi A, Yeo G. ISUOG Practice Guidelines: performance of first‐trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 2013; 41: 102–113.
4. Papaioannou, George I., et al. "Normal ranges of embryonic length, embryonic heart rate, gestational sac diameter and yolk sac diameter at 6–10 weeks." Fetal diagnosis and therapy28.4 (2010): 207-219.
5. Rodgers, Shuchi K., et al. "Normal and abnormal US findings in early first-trimester pregnancy: review of the society of radiologists in ultrasound 2012 consensus panel recommendations." Radiographics 35.7 (2015): 2135-2148.