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:
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
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
- 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.
- 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.
- 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.
- 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.
- 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).
- 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.
M-mode ultrasound shows a heart rate of 111 beats/min of 6w 6D embryo.
The placenta early appears in 8-9weeks of gestation
Uterus: Normal size and echotexture Ovaries: Right ___ / Left ___ (corpus luteum, cysts if any) Adnexa: No adnexal masses Free Fluid: None / Minimal physiological
| 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].
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].