Anatomy and Physiology
The uterus is bordered by the bladder anteriorly and the rectum posteriorly. In most women, the uterus is anteverted, which indicates that the uterus is superior/posterior to the bladder and anterior to the rectum, with the uterine fundus more anteriorly oriented. Women may also have anteflexed, retroverted, retroflexed uterine orientations.[7]
The uterus has a hyperechoic stripe in the center of the fundus and communicates with the cervix. Distal to the cervix is the vaginal canal, which appears as a hyperechoic line when viewed with a transabdominal transducer.
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In the adnexa to either side of the uterus, the right and left ovaries classically lie between the lateral uterine wall and the internal iliac vessels, a relationship more likely altered in multiparous women. The adnexa also includes the fallopian tubes connecting the uterus and ovaries, both of which should be carefully evaluated as they are the most likely locations of ectopic pregnancies.[8] Ovaries classically have a slightly hypoechoic echotexture compared to surrounding tissue and a “chocolate chip cookie” characteristic due to peripheral, anechoic follicles. Large corpus luteum cysts are commonly seen in early pregnancy, typically as a single large, simple cystic structure within ovarian tissue. Corpus luteum cysts involute as the placenta develops to support the growing fetus.
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The first sign of intrauterine pregnancy is the gestational sac, an anechoic structure that appears between the fourth and fifth gestational weeks but is small at 2 or 3 mm in the greatest dimension.[8] Although not always seen, the double decidual sign is the first widely accepted sign of intrauterine pregnancy.[8][9] It is composed of two echogenic rings, the decidua capsularis (inner ring) and decidua vera (outer ring), which surround the developing gestational sac.[10][8] Because the double decidual sac sign may be confused with a pseudogestational sac (nonviable), many clinicians elect to visualize a yolk sac before confirming an intrauterine pregnancy. The yolk sac is typically visible by ultrasound in the fifth week and appears as a hyperechoic ring with an anechoic center located within the gestational sac.[8][11]
A fetal pole can be visualized in the sixth gestational week and appears as a hypoechoic structure immediately adjacent to the yolk sac.[8][11] By the sixth or seventh gestational week, fetal cardiac activity is usually evident.[8] In the seventh week, the amnion becomes sonographically visible and appears as a larger echogenic ring within the fetal pole’s gestational sac.[11] The amnion expands after fetal urine production begins in the 10th week and fuses with the chorion by 14 to 16 weeks.[11]
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