Identifying an Intrauterine Pregnancy
To definitively diagnose an intrauterine pregnancy, either a yolk sac or a fetal pole must be seen within a gestational sac inside of the uterus.
|Transvaginal Ultrasound||Transabdominal Ultrasound|
|Gestational sac||4-5 weeks||Not reliable for gestational age under 6 weeks|
|Yolk sac||5 weeks||6-7 weeks|
|Fetal pole||5-5.5 weeks||7 weeks|
Gestational Sac (4-5 weeks)
The gestational sac is a collection of fluid surrounding the embryo and yolk sac. It is the first structure to be seen in the development of an IUP. It will have these features:
- Anechoic (dark), round structure with an echogenic (bright) border.
- Typically, it is in the upper 1/3 of the uterine fundus.
- You should expect to see the gestational sac at 4.5-5 weeks gestational age with transvaginal ultrasound.
- Make sure the sac is actually in the uterus by tracking the vaginal stripe to the uterus.
Shortly after the appearance of the gestational sac, the decidual layers are commonly seen. This is called the Double Decidual Sign. They will appear as 2 echogenic rings surrounding the gestational sac. The outer ring is the decidua parietalis (lining the uterine cavity) and the inner ring the decidua capsularis (lining the gestational sac). The presence of a Double Decidual Sign is highly indicative of an early intrauterine pregnancy. However, a definitive diagnosis of IUP will require the presence of a yolk sac or fetal pole.
Tip: Don’t diagnose a IUP until you see a yolk sac or fetal pole within a uterine gestational sac.
A common pitfall is to falsely identify a gestational sac. Small collections of fluid can look very similar to gestational sacs and are appropriately called Pseudogestational sacs. How can we tell them apart? Pseudogestational sacs will usually have one or more of the following characteristics:
- A pseudogestational sac is more irregularly shaped or pointy-edged than a round gestational sac.
- The border surrounding the sac is not as echogenic as that of a true gestational sac.
- The fluid within a pseudo-gestational sac is not completely anechoic, there are some echoes seen in the fluid.
- The fluid of a pseudo-gestational sac will not be found in the decidua like a true gestational sac, but in the uterine cavity.
- A pseudo-gestational sac will not have the contents of a maturing gestational sac such as the yolk sac and embryo.
Correctly distinguishing between a true and pseudo-gestational sac is important because intrauterine fluid collections reportedly occur in 9-20% of ectopic pregnancies. Additionally, misdiagnosing a pseudo-gestational sac for an early IUP can lead to improper treatment.
Note the irregular shape, lack of double decidual sign, presence of echoes in the fluid, and lack of embryonic contents in Pseudogestational Sac.
Yolk Sac – (5 weeks)
Typically seen at around 5 weeks gestational age by transvaginal OB ultrasound, the yolk sac is an early source of nutrition for the developing embryo which usually isn’t yet visible. The yolk sac is a circular, echogenic ring with an anechoic center seen eccentrically (not in the center) in the gestational sac.
Fetal Pole – (5.5-6 weeks)
The fetal pole, or developing embryo, should be seen at 5.5-6 weeks gestational age by transvaginal ultrasound. It grows directly adjacent to the yolk sac. With transvaginal ultrasound, the fetal pole should be seen when it is 2-4mm in length.