Early first trimester normal ultrasound survey

Normal first trimester ultrasound survey

Understand critical signs, scan parameters, and interpretation tips. Ideal for OB-GYN and radiology learners.
The first trimester of pregnancy (Less than 13w) is a critical period of development, during which the embryo progresses to a fetus. Here’s an overview of the normal anatomy observed during this stage:

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Early first trimester structure (Less than 10w 06d)
1– Intradecidual Sac Sign (IDSS)/Very early pregnancy
2– Gestational Sac
3– Yolk Sac
4– Amnion
5– Fetal Pole
6– Cardiac Activity
7– Omphalomesenteric duct or vitelline duct
8– Placenta
9– Uterus and Adnexa
10– Timeline of fetal structure appears in ultrasound TAS/TVS
11– Differences in Detection: TAS vs TVS

1– Intradecidual Sac Sign (IDSS)/Very early pregnancy

The intradecidual sac sign (IDSS) is one of the earliest ultrasound findings suggestive of an intrauterine pregnancy (IUP). It helps distinguish an early intrauterine gestational sac from a potential ectopic pregnancy.

• The IDSS appears as a small, round, anechoic (fluid-filled) structure within the decidua of the endometrial cavity, but not in the central endometrial stripe.
• It is typically seen as early as 4 to 4.5 weeks of gestation on transvaginal ultrasound (TVUS).
• The sac is located eccentrically, meaning it is implanted within one side of the thickened decidua rather than in the middle

Significance:

• Early indicator of intrauterine pregnancy (IUP): The IDSS is considered an early sign before the yolk sac and fetal pole become visible.
• Distinguishing from pseudogestational sac: Unlike a central fluid collection seen in ectopic pregnancies (pseudogestational sac), the true gestational sac appears eccentric in location with a surrounding echogenic rim.
• Confirmation needed: IDSS alone is not definitive for a viable pregnancy and should be followed up with serial ultrasounds to confirm the presence of a yolk sac, fetal pole, and cardiac activity.

2– Gestational Sac

• The first identifiable structure on ultrasound, typically visible by 4.5 to 5 weeks via transvaginal ultrasound and in trans abdominal at 5 weeks 1 day of conception
• Located in the fundal or mid-uterine cavity.
• Grows approximately 1 mm per day.

3– Yolk Sac

• Appears within the gestational sac around 5 weeks.
• Provides early nutrition and contributes to blood cell formation.
• Should be ≤ 6 mm; a larger size may indicate abnormal pregnancy.

4– Amnion

The amnion is the thin, inner fetal membrane that encloses the amniotic cavity, which contains the amniotic fluid and surrounds the developing embryo/fetus.

It plays a vital role in:

• The amnion early appears at 7w in sonogram.
• Providing a protective fluid environment.
• Allowing fetal movement and growth.
• Preventing adhesions between the embryo and surrounding structures.
• Serving as a barrier against infections.

5– Fetal Pole

• Visible by 5.5 to 6 weeks on transvaginal ultrasound.
• Grows at approximately 1 mm per day.
• Crown-rump length (CRL) is used to estimate gestational age.
• Cardiac activity should be present by 6 weeks (normal heart rate: 100–180 bpm).

6– Cardiac Activity
Early cardiac pulsation appears 5w 6D to 6w 3D.
M-mode ultrasound shows a heart rate of 111 beats/min of 6w 6D embryo.

7– Omphalomesenteric duct or vitelline duct

• The omphalomesenteric duct (OMD), also known as the vitelline duct, is an embryological structure that connects the midgut to the
• yolk sac. Early appears at 6w.
• Begins forming around 5–6 weeks.
• Initially short, later elongates with twisting due to fetal movements.
• Contains two arteries and one vein.

8– Placenta

The placenta early appears in 8-9weeks of gestation

9– Uterus and Adnexa
Uterus: Normal size and echotexture Ovaries: Right ___ / Left ___ (corpus luteum, cysts if any) Adnexa: No adnexal masses Free Fluid: None / Minimal physiological

10– Timeline of fetal structure appears in ultrasound TAS/TVS

Gestational Age (GA)	Structure Seen	Transvaginal Scan (TVS)	Transabdominal Scan (TAS)
3.5–4 weeks	Endometrial decidual reaction	✅ Yes	⚠️ Rare
4–4.5 weeks	Gestational sac (GS)	✅ Visible (2–3 mm)	⚠️ Difficult to see
5.0 weeks	Yolk sac (within GS)	✅ Seen clearly	⚠️ May be faint or absent
5.5–6.0 weeks	Fetal pole	✅ Detectable	⚠️ Often not seen
6.0 weeks	Cardiac activity (flicker)	✅ Yes (with fetal pole)	⚠️ Often missed
6.5–7.0 weeks	Embryo length (CRL measurable)	✅ Clearly seen	⚠️ May be difficult
7–8 weeks	Amniotic sac, head-body differentiation	✅ Good detail	⚠️ Limited view
8–9 weeks	Limb buds, developing brain vesicles	✅ Yes	✅ Possibly visible
9–10 weeks	Facial features, limb movement, spinal outline	✅ Yes	✅ Yes
11–13+6 weeks	NT, nasal bone, detailed anatomy survey	✅ Excellent resolution	✅ Adequate for screening

Notes:
1– TVS is more sensitive and should be used for early pregnancy Less than 8 weeks.
2– TAS is better after the bladder is full and GA is >7 weeks.
3– Cardiac activity should be confidently detected by 6.0–6.3 weeks (TVS).
4– If no fetal pole by MSD ≥25 mm or no cardiac activity in CRL ≥7 mm consider nonviability [based on current ACOG (American College of Obstetricians and Gynecologists) criteria].

11– Differences in Detection: TAS vs TVS

Feature/Aspect	Transvaginal Scan (TVS)	Transabdominal Scan (TAS)
Proximity to uterus	Very close – probe inserted vaginally	Further away – probe placed on abdomen
Image resolution	Higher resolution, clearer in early pregnancy	Lower resolution in early stages
Earliest gestational sac visibility	4.0 – 4.5 weeks GA	5.0 – 5.5 weeks GA
Yolk sac visibility	Seen from ~5.0 weeks	May be faint or not seen
Fetal pole detection	Clear from 5.5–6 weeks	Often not detected until ~6.5+ weeks
Cardiac activity detection	From 6.0 weeks (as early flicker)	Often missed before 7 weeks
CRL (Crown-Rump Length) measurement	From 6.5 weeks accurately	Less reliable before 8 weeks
Visualizing pelvic structures	Excellent for ovaries, uterus, cervix	Limited pelvic detail
Maternal habitus impact	Minimal interference	Affected by maternal body fat and bladder filling
Patient comfort	Invasive; requires consent and may cause discomfort	Non-invasive; more comfortable
Bladder requirement	Empty bladder required	Full bladder improves visibility