Mean Sac Diameter Calculator

This mean sac diameter calculator is used in obstetrics for assessing early pregnancy development by measuring the size of the gestational sac, the fluid-filled structure that surrounds the embryo during the initial weeks of pregnancy.

• An ultrasound technician measures three perpendicular diameters of a gestational sac: 15mm, 16mm, and 17mm. The MSD Calculator averages these measurements to determine the mean sac diameter.
• A doctor utilizes the MSD Calculator to monitor the growth of a gestational sac across multiple ultrasound appointments, ensuring proper development.

Mean Sac Diameter Calculator

Gestational Age	Measurement 1	Measurement 2	Measurement 3	Mean Sac Diameter
3 weeks	3mm	3mm	4mm	3.33mm
4 weeks	4mm	5mm	4mm	4.33mm
5 weeks	6mm	7mm	8mm	7mm
5 weeks	9mm	8mm	7mm	8mm
6 weeks	14mm	15mm	16mm	15mm
6 weeks	12mm	13mm	14mm	13mm
7 weeks	22mm	23mm	24mm	23mm
7 weeks	20mm	21mm	22mm	21mm
8 weeks	30mm	31mm	32mm	31mm
8 weeks	28mm	29mm	30mm	29mm
9 weeks	38mm	39mm	40mm	39mm
9 weeks	36mm	37mm	38mm	37mm
10 weeks	45mm	46mm	47mm	46mm
10 weeks	42mm	43mm	44mm	43mm
11 weeks	50mm	51mm	52mm	51mm
11 weeks	48mm	49mm	50mm	49mm
12 weeks	60mm	61mm	62mm	61mm
12 weeks	58mm	59mm	60mm	59mm
13 weeks	65mm	66mm	67mm	66mm
13 weeks	63mm	64mm	65mm	64mm
14 weeks	70mm	71mm	72mm	71mm
14 weeks	68mm	69mm	70mm	69mm

Mean Sac Diameter Formula

The formula for calculating Mean Sac Diameter is:

MSD = (Length + Width + Height) / 3

Where:

• Length, Width, and Height are the three perpendicular measurements of the gestational sac in millimeters.

If the measurements are:

• Length: 18mm
• Width: 20mm
• Height: 19mm

MSD = (18 + 20 + 19) / 3 = 19mm

How do you calculate mean sac diameter?

1. Obtain three perpendicular measurements of the gestational sac using ultrasound imaging.
2. Ensure measurements are in millimeters.
3. Add the three measurements together.
4. Divide the sum by 3 to get the average.

Ultrasound measurements:

• Measurement 1: 22mm
• Measurement 2: 24mm
• Measurement 3: 23mm

Calculation:

Sum: 22 + 24 + 23 = 69mm

MSD: 69 / 3 = 23mm

The mean sac diameter in this case is 23mm.

What is the mean sac diameter at 6 weeks?

At 6 weeks gestation, the typical mean sac diameter ranges from 12 to 18 millimeters.

A woman at exactly 6 weeks gestation has an ultrasound. The technician measures the gestational sac:

• Measurement 1: 14mm
• Measurement 2: 15mm
• Measurement 3: 16mm

MSD = (14 + 15 + 16) / 3 = 15mm

This 15mm MSD falls within the expected range for 6 weeks gestation.

What is the mean sac diameter at 5 weeks?

At 5 weeks gestation, the mean sac diameter typically ranges from 5 to 10 millimeters.

A woman believed to be 5 weeks pregnant undergoes an ultrasound. The gestational sac measurements are:

• Measurement 1: 7mm
• Measurement 2: 8mm
• Measurement 3: 7mm

MSD = (7 + 8 + 7) / 3 = 7.33mm

This MSD of 7.33mm is consistent with a 5-week gestation.

What is the mean sac diameter of a miscarriage?

• An empty gestational sac measuring >25mm
• A sac that doesn’t grow appropriately between scans
• A sac that’s significantly smaller than expected for the gestational age

A woman at 7 weeks gestation has an ultrasound showing:

• Measurement 1: 10mm
• Measurement 2: 11mm
• Measurement 3: 10mm

MSD = (10 + 11 + 10) / 3 = 10.33mm

This MSD is considerably smaller than the expected 20-30mm for 7 weeks, potentially indicating a non-viable pregnancy.

What is the diameter of the sac at 3 weeks?

At 3 weeks post-conception (considered 5 weeks gestation due to pregnancy dating), at this early stage is usually between 2-4 millimeters.

An early ultrasound at 3 weeks post-conception shows:

• Measurement 1: 3mm
• Measurement 2: 3mm
• Measurement 3: 4mm

MSD = (3 + 3 + 4) / 3 = 3.33mm

This MSD of 3.33mm is consistent with very early pregnancy development.

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