Basic principles and early weeks of gestation
Ultrasound examination is a safe, non-invasive, and highly informative tool that does not require special preparation from a pregnant woman. Two main approaches are used in obstetric practice: transabdominal and transvaginal scanning.
Imaging dynamics in the earliest stages:
- 3 weeks: The gestational sac begins to be detected in the uterine cavity. It appears as an echonegative round or ovoid structure. While the embryo is not yet visible, the gestational age is calculated from the mean sac diameter.
- 4–5 weeks: The device allows visualization of the yolk sac (a ring-like structure) and the embryonic disc itself, which is visualized as an echopositive strip measuring 6–7 mm.
Assessment of development and viability
Throughout the first trimester, the key landmark for establishing accurate gestational age is the crown-rump length (CRL). In addition to measurements, the physician carefully assesses signs of viability.
Heart rate (HR) changes non-linearly:
- At 5–6 weeks, the normal rate is 150–160 beats per minute.
- By 7–8 weeks, the rhythm gradually accelerates to 175–185 beats.
- Approaching the 12th week, the rate smoothly drops back to 150–160 beats per minute.
Furthermore, starting from 7–8 weeks, the specialist can record the first signs of embryonic motor activity.
Ultrasound criteria for non-viable pregnancy
If embryonic demise occurs, but clinical signs of miscarriage are absent, a non-viable (missed) pregnancy is diagnosed. Ultrasound distinguishes two variants: demise of the embryo itself inside the uterus and anembryony (absence of an embryo, resulting in the formation of only an empty gestational sac).
The primary diagnostic tool in such cases is transvaginal ultrasound. An absolutely reliable criterion for demise is the absence of cardiac activity, provided that the CRL exceeds 7 mm.
If the ultrasound is not performed in time, the demised gestational sac is retained in the uterus (pathological retention). This occurs due to deep invasion of chorionic villi, a defective shedding reaction, or chronic endometritis, which lacks receptors for endogenous uterotonics. The tissues undergo necrosis, maceration, and, in early stages, autolysis (more rarely, the embryo becomes mummified or calcified). Clinically, the woman may notice the disappearance of presumptive signs of pregnancy, lack of uterine growth, and spotting caused by detachment of the gestational sac.
Imaging challenges in rare forms of ectopic pregnancy
Sometimes the ultrasound picture is inconclusive due to rare forms of ectopic pregnancy. For example, in ovarian pregnancy, chorionic villi invade directly into the ovarian tissue, and the amnion and chorion create a capsule. On the monitor, this often looks like a progressive tubal pregnancy, and the exact diagnosis is made only intraoperatively (usually after rupture of the gestational sac at 6–8 weeks).
It is even more difficult to detect pregnancy in a rudimentary (accessory) uterine horn. On ultrasound, the gestational sac in this area is extremely difficult to identify, as the picture mimics a normal intrauterine pregnancy. This pathology requires surgical removal of the horn due to the colossal risk of massive intraperitoneal hemorrhage, associated with the blood supply to the horn from the uterine artery.
Transition to the second and third trimesters: Fetometry
After the completion of the first trimester, the focus of the ultrasound examination shifts. In the second and third trimesters, fetometry comes to the forefront—measuring fetal parameters to assess growth and estimate fetal weight.
The physician necessarily records the following main parameters:
- Biparietal diameter (BPD).
- Head circumference (HC).
- Abdominal circumference (AC).
- Femur length (FL).
To memorize the heart rate dynamics in the first trimester, use the "150 — 180 — 150" rule. At 5–6 weeks, the heart beats at 150–160 bpm, accelerates to 175–185 bpm by 7–8 weeks, and calms back down to 150–160 bpm by the end of the trimester (12 weeks).
Class and exam questions
At what minimum gestational age does ultrasound show the gestational sac?
An echonegative gestational sac begins to be visualized in the uterine cavity from the 3rd week of pregnancy.
What is the most accurate way to determine gestational age in the first trimester?
Before the appearance of the embryo, gestational age is determined by the mean sac diameter. Once the embryo becomes visible, the age is established by measuring its crown-rump length (CRL).
Which ultrasound sign unequivocally indicates a missed miscarriage?
A reliable criterion for demise is the complete absence of embryonic cardiac activity if the crown-rump length (CRL) is greater than 7 mm.
What else to study on this topic
- Clinical presentation and principles of treatment for septic abortion
- Management tactics and hCG monitoring after surgery for ectopic pregnancy
- Rehabilitation and contraception after organ-sparing tubal surgeries
- Savelyeva's scoring system: assessment of fetal cardiac activity in the first and second stages of labor
- Dangers and consequences of illegal termination of pregnancy
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