1st Trimester Parameters & Reference Values

Early 1st Trimester Parameters & Reference Values

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Early 1st Trimester Measuring Parameters

1. MSD (Mean Sac Diameter)
2. GS Volume
3. CRL (Crown-Rump Length)
4. HR (Heart Rate)

1. MSD (Mean Sac Diameter)

Estimation of gestational age from MSD is used when the embryo has not yet appeared. MSD is calculated by averaging the sac's dimensions:

Formula: Mean sac diameter (cm) = [L + AP + T] / 3

Example:
MSD = (1.5 cm + 0.9 cm + 1 cm) / 3 = 3.4 cm / 3 = 11.34 mm
Menstrual age (days) = MSD (mm) + 30 = 11.34 + 30 = 41.34 days = 5.9 weeks ≈ 5w6d

MSD increases ~1.1 mm per day.

2. GS Volume

Also used only when the embryo is not yet visible.
Formula: GS Volume (ml) = L × T × AP × 0.5233

3. CRL (Crown-Rump Length)

Measurement from crown to rump. It is the most accurate parameter for dating in the 1st trimester once the embryo is visible.

4. HR (Heart Rate)

• Use M-mode or PW Doppler
• CRL > 10 mm (TA) without heart pulsation → Abnormal
• CRL > 7 mm (TV) without heart pulsation → Abnormal

Early 1st Trimester Reference Values

Intradecidual Sac: 1–2 mm at 4.5 weeks
GS: 4 mm at 4.5 weeks

Gestational Sac (GS) Reference Table:

GS (mm)	GA (weeks)	GS (mm)	GA (weeks)
10	5.0	36	8.8
11	5.2	37	8.9
12	5.3	38	9.0
13	5.5	39	9.2
14	5.6	40	9.3
15	5.8	41	9.5
16	5.9	42	9.6
17	6.0	43	9.7
18	6.2	44	9.9
19	6.3	45	10.0
20	6.5	46	10.2
21	6.6	47	10.3
22	6.8	48	10.5
23	6.9	49	10.6
24	7.0	50	10.7
25	7.2	51	10.9
26	7.3	52	11.0
27	7.5	53	11.2
28	7.6	54	11.3
29	7.8	55	11.5
30	7.9	56	11.6
31	8.0	57	11.7
32	8.2	58	11.9
33	8.3	59	12.0
34	8.5	60	12.2
35	8.6

Early First Trimester HR Reference Table (with Bradycardia & Tachycardia):

Gestational Age (Weeks)	Expected Heart Rate (bpm)	Clinical Interpretation
5.5 – 6.0	90 – 110 bpm	Lower end of normal; may increase rapidly
6.0 – 7.0	110 – 130 bpm	Normal developing embryo
7.0 – 8.0	130 – 160 bpm	Peak heart rate zone
8.0 – 9.0	140 – 170 bpm	Maintained high normal range
>9.0	120 – 160 bpm	Stable fetal heart rate
Any age	< 90 bpm	Fetal Bradycardia: May be associated with poor outcomes if persistent, especially below 85 bpm at 6–7 weeks
Any age	> 180 bpm	Fetal Tachycardia: May suggest fetal distress, infection, or arrhythmia; follow-up needed

CRL Reference Table – First Trimester (6–13 Weeks):

CRL (mm)	Gestational Age (Weeks + Days)	Expected Fetal Heart Activity
1 – 2 mm	5w0d – 5w3d	May not yet be visible
3 – 4 mm	5w4d – 6w0d	Possible early flicker
5 – 7 mm	6w1d – 6w6d	FHR usually detectable
8 – 12 mm	7w0d – 7w6d	Normal rhythmic cardiac activity
13 – 20 mm	8w0d – 9w6d	Consistent FHR expected
21 – 30 mm	10w0d – 11w6d	Well-formed fetus; strong FHR
31 – 40 mm	12w0d – 13w6d	FHR and fetal movement visible

Components of Documentation – 3.141

• IUP or no IUP
• Gestation: Single, Twins, Triplets, or Higher-order
• FCA or no FCA
• Gestational age
• Uterine masses
• Adnexal masses
• Free fluid

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Topic Related MCQ

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Note: If you select English, answer all questions in English.
यदि आप हिंदी चुनते हैं, तो सभी प्रश्न हिंदी में हल करें।

1. What does MSD stand for in early pregnancy ultrasound? 1. प्रारंभिक गर्भावस्था अल्ट्रासाउंड में MSD का क्या अर्थ है?
A. Mean Stomach Diameter
B. Median Sac Diameter
C. Mean Sac Diameter
D. Minimal Systolic Diameter

2. Which formula is used to calculate the Mean Sac Diameter (MSD)? 2. Mean Sac Diameter (MSD) की गणना के लिए कौन सा सूत्र प्रयोग होता है?
A. L × T × AP
B. (L + T + AP) / 3
C. L + T + AP
D. L² + T² + AP²

3. What is the gestational age formula using MSD in mm? 3. MSD (mm) का उपयोग करके गर्भावधि आयु की गणना कैसे की जाती है?
A. MSD × 2
B. MSD + 30
C. MSD / 7
D. MSD – 10

4. At what rate does the MSD typically increase per day? 4. MSD प्रति दिन सामान्यतः कितनी वृद्धि करता है?
A. 0.5 mm/day
B. 2 mm/day
C. 1.1 mm/day
D. 3 mm/day

5. GS volume is calculated using which formula? 5. GS वॉल्यूम किस सूत्र से मापा जाता है?
A. L × W × H × 0.42
B. (L + T + AP)/3
C. L × T × AP × 0.5233
D. L + T + AP

6. What is the most reliable parameter for dating in the first trimester? 6. पहले त्रैमासिक में डेटिंग के लिए सबसे विश्वसनीय पैरामीटर कौन सा है?
A. Gestational sac
B. Heart rate
C. Crown-Rump Length (CRL)
D. Yolk sac

7. What is considered abnormal if CRL is >7 mm and no cardiac activity is detected on TV? 7. यदि CRL >7 mm है और TV पर कोई हृदय गति नहीं है तो क्या असामान्य माना जाता है?
A. Normal variation
B. Ectopic pregnancy
C. Subchorionic hematoma
D. Early pregnancy failure

8. Bradycardia is defined as fetal heart rate less than: 8. ब्रैडीकार्डिया को भ्रूण की हृदय दर कितनी से कम पर परिभाषित किया जाता है?
A. 120 bpm
B. 90 bpm
C. 100 bpm
D. 80 bpm

9. At what gestational week does CRL measurement become most accurate? 9. CRL मापन किस गर्भ सप्ताह में सबसे सटीक होता है?
A. 10 weeks
B. 7 weeks
C. 11–13 weeks
D. 6 weeks

10. Tachycardia in the first trimester is defined as fetal heart rate above: 10. पहले त्रैमासिक में टैकीकार्डिया को भ्रूण की हृदय दर कितनी से अधिक पर परिभाषित किया जाता है?
A. 130 bpm
B. 140 bpm
C. 160 bpm
D. 120 bpm

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