First trimester ultrasound measuring method

 First Trimester Ultrasound Section structure, scanning technique & Measuring Methods

1. Gestational Sac Diameter (GSD)

• Definition: Measures the mean sac diameter (MSD) by averaging three perpendicular measurements of the gestational sac. The MSD increases in size at a rate of 1.1 mm per day.

• Use: Typically used before the embryo is visible, around 4 to 6 weeks gestation.
• Calculation:
  • MSD = (Length + Width + Height) / 3
  • Estimated gestational age = MSD (mm) + 30 days
• Limitations: Less accurate than CRL; once the embryo is visible, CRL is preferred.

➤Formula for Gestational Age (GA):
Estimation of gestational age from mean sac diameter (MSD). This method is only used when the fetal pole/ embryo has not yet appeared. The MSD is an average of the diameter of the sac, obtained by adding the anteroposterior and craniocaudal diameters on the sagittal view of the uterus to the transverse diameter obtained on the transverse view and dividing by three.

If Gestational Sac measurements are:

• Length (L) = 20 mm
• Width (W) = 18 mm
• Height (H) = 22 mm

Then:

Accuracy: Less reliable than CRL; used before the fetal pole is visible (4-6 weeks).

Example:

Step 1: Calculate Mean Sac Diameter (MSD)

Formula:

$$MSD = \frac{L + AP + T}{3}$$

Given measurements:

• L (Length) = 1.5 cm
• AP (Anteroposterior) = 0.9 cm
• T (Transverse) = 1.0 cm

$$MSD = \frac{1.5 + 0.9 + 1.0}{3} = \frac{3.4}{3} = 1.13 \text{ cm} = 11.34 \text{ mm}$$

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Step 2: Calculate Menstrual Age in Days

Formula:

$$\text{Menstrual Age (days)} = MSD \text{ (mm)} + 30$$

$$= 11.34 + 30 = 41.34 \text{ days}$$

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Step 3: Convert to Weeks & Days

Formula:

$$\text{Menstrual Age (weeks)} = \frac{\text{Menstrual Age (days)}}{7}$$

​ $$= \frac{41.34}{7} = 5.9 \text{ weeks} = 5 \text{ weeks } 6 \text{ days (5w6d)}$$

➤Formula for Gestational Volume: Measurement used to estimate early pregnancy age. The most commonly used formula for calculating gestational sac volume (GSV) is:

Formula for Gestational Sac Volume (GSV)

$$GSV = \frac{L \times AP \times T}{2}$$

$$Or\; Gestation\; sac\; volume\; (ml)=\; L(cm)\; \times \; T(cm)\times \; AP\; (cm)\; \times \; 0.5233$$

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Where:

• L = Length (cm)
• AP = Anteroposterior diameter (cm)
• T = Transverse diameter (cm)

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Example Calculation

Given:

• L = 1.5 cm
• AP = 0.9 cm
• T = 1.0 cm

$$GSV = \frac{(1.5 \times 0.9 \times 1.0)}{2}$$ $$= \frac{1.35}{2}$$ $$= 0.675 \text{ cm}^3$$

Gestational Sac Volume (GSV) = 0.675 cm³

➤Scanning technique, section structures and measuring methods

Scanning Method: 

Longitudinal Scanning of Early Pregnant Uterus: The patient is maintained in the supine position, with full bladder and exposure of the lower abdomen. The probe should be placed longitudinally on the medioventral line above the pubic symphysis.

Measuring Method: The longitudinal diameter and the anteroposterior diameter of uterus are measured in this section, as well as the longitudinal diameter and the anteroposterior diameter of the gestational sac.

MSD >10mm must have a yolk sac.

MSD >18mm must have an embryo.

CRL >5mm must have a heartbeat.

Section Structure: Sagittal section of the uterus and sagittal section of the gestational sac.

1. GS- Gestational sac
2. BL- Bladder
3. Ut- Uterus

 The Clinical Application Value:

Bilayer structure is an important character of the early gestational sac, which consisted of the inner layer of amnion and the outer layer of chorion, shown as two slightly strong echoic rings with a hypoechoic region between the two rings. The size, shape, and tension of the gestational sac should be observed also.

Scanning Method:

Transverse Scanning of Early Pregnant Uterus: The patient is maintained in the supine position, with full bladder and exposure of the lower abdomen. The probe should be placed transversely on the medioventral line above the pubic symphysis.

Measuring Method: The transverse diameter of the uterus and the transverse diameter of the gestational sac are measured in this section

Section Structure: Transverse section of the uterus and transverse section of the gestational sac.

1. GS- Gestational sac
2. BL- Bladder
3. Ut- Uterus

The Clinical Application Value:

The size, shape, and tension of the gestational sac can be observed in this section, as well as the left and right adnexal regions including the corpus luteum and its size.

➤Gestational Sac (GS) Size by Weeks of Gestation

Gestational Age (Weeks)	Mean Sac Diameter (MSD) (mm)	Range (mm)
4 weeks	2 mm	1–4 mm
5 weeks	5 mm	4–8 mm
6 weeks	10 mm	7–14 mm
7 weeks	16 mm	11–22 mm
8 weeks	25 mm	18–30 mm
9 weeks	35 mm	27–40 mm
10 weeks	45 mm	35–50 mm

➤Key Points:

• The gestational sac is the first sign of pregnancy visible on ultrasound, typically at 4.5 to 5 weeks.
• The GS normally grows about 1 mm per day in early pregnancy.
• A yolk sac should be visible when the MSD reaches 10 mm.
• A fetal pole and heartbeat should be detected by 5.5–6 weeks.
• If the MSD reaches ≥25 mm without an embryo, it may indicate a blighted ovum (anembryonic pregnancy).

2. Yolk Sac Measurement

• Measures the diameter of the yolk sac.

• Use: Helps assess early pregnancy health; abnormal size (>6mm or irregular shape) may indicate pregnancy complications.

Yolk Sac Measurement by Gestational Weeks

Here’s an approximate guide to normal yolk sac size by gestational age:

➤Gestational Age (Weeks)	Yolk Sac Diameter (mm)
5 Weeks	2.0 – 3.0 mm
5.5 Weeks	3.0 – 4.0 mm
6 Weeks	4.0 – 5.0 mm
7 Weeks	5.0 – 6.0 mm
8 Weeks	Begins to regress
10 Weeks	Usually disappears

➤Key Points About Yolk Sac Measurement:

1. Normal Size Range: A normal yolk sac is typically 2–6 mm in diameter.
2. Too Small (<2 mm): May indicate early pregnancy failure.
3. Too Large (>6 mm): Associated with an increased risk of miscarriage or chromosomal abnormalities.
4. No Yolk Sac: By 5.5-6 weeks, a missing yolk sac could indicate a non-viable pregnancy.
5. Regression: The yolk sac usually disappears by 10–12 weeks as the placenta takes over nourishment.

3. Crown-Rump Length (CRL)

• Definition: Measures the length of the embryo from the top of the head (crown) to the bottom of the torso (rump).
• Procedure:
• The fetus is measured in a neutral position (not curled up or stretched out).
• The longest straight-line distance is recorded.
• Measurement is taken three times, and the average is used.

➤Formula for Gestational Age (GA):

Robinson & Fleming (1975) Formula: 

Hadlock Formula (1992):  

➤ CRL Size by Weeks of Gestation:

CRL (mm)	GA (weeks)

1-5 mm

5-6 weeks

6-10 mm

6-7 weeks

11-15 mm

7-8 weeks

16-22 mm

8-9 weeks

23-32 mm

9-10 weeks

33-41 mm

10-11 weeks

42-54 mm

11-12 weeks

55-65 mm

12-13 weeks

4. Fetal Heart Rate (FHR)

• Measured by: M-mode ultrasound to assess viability.
• Normal Range:
• 100–120 bpm at 6 weeks
• 120–160 bpm at 7+ weeks

Gestational Age Based on Fetal Heart Rate (FHR)

Formula (approximation):

Example:
If FHR = 140 bpm, then:

Accuracy: Used as an additional parameter; normal range is 100-160 bpm depending on GA.

5. Nuchal Translucency (NT)

• Definition: Measures fluid at the back of the fetal neck between 11-14 weeks.
• Use: Screening for chromosomal abnormalities (e.g., Down syndrome).