Input three dimensions of the gestational sac, the mean sac diameter or the CRL to calculate estimated gestational age and expected date of delivery (EDD).
Estimated Gestational Age (MSD):
Estimated Gestational Age (CRL1):
Estimated Gestational Age (CRL.2):
CRL Age estimation 1 is based on Robinson & Fleming formula(1975) while CRL Age estimation 2 is based on the formula by Tan et. al. (2023)
Key Takeaways
- Calculating gestational age accurately during early pregnancy is crucial for monitoring the baby’s growth and development.
- The crown rump length (CRL) and mean sac diameter measurements are two commonly used methods for estimating gestational age in the first trimester.
- CRL measures the length of the baby from the top of the head to the bottom of the buttocks, usually between 7 and 13 weeks of gestation.
- Mean sac diameter is the average measurement of the gestational sac surrounding the embryo, typically taken between 5 and 10 weeks of gestation.
- Both measurements are obtained through ultrasound examinations and are used in conjunction with the date of the last menstrual period to provide the most accurate assessment of gestational age.
Gestational Age Calculator: Importance of Accurate Measurements
What is Gestational Age?
Gestational age, or how far along a pregnancy is, plays a vital role in monitoring the baby’s growth and development. Determining gestational age accurately is crucial for several reasons, including monitoring the baby’s progress, identifying potential issues or complications early on, and planning appropriate prenatal care and interventions if necessary.
Crown-Rump Length (CRL) Calculation
The crown-rump length (CRL) is a measurement of the embryo from the top of the head (crown) to the bottom of the buttocks (rump). This measurement is most accurate between 7 and 13 weeks of gestation, when the embryo is still too small to accurately measure other anatomical features.
How to measure CRL:
During an early pregnancy ultrasound, a trained healthcare professional will use a transducer to create ultrasound images of the embryo. The transducer emits sound waves that bounce off the baby and internal structures, creating an image on the screen. The healthcare professional will then use calipers on the ultrasound screen to measure the CRL.
How to calculate gestational age from CRL?
You can do this by using formulas or charts.
Gestational Age from CRL using formulas:
These are the formulas used in this gestational age calculator:
- Estimated Gestational Age in days using Robinson & Fleming formula = 8.052 × √(CRL × 1.037) + 23.73
- Estimated Gestational Age in days using Tan et. al formula = 37.31 + (1.39 × CRL) − (0.014 × CRL2)+ (0.00007 × CRL3)
The formula by Tan et. al is the more recent (2023) whereas the one by Robinson and Fleming has been published in 1975. The difference is only a few days and either measurement should be fine.
Gestational Age from CRL using Charts:
You can use the following charts to estimate fetal age from CRL:
| Crown-Rump Length (CRL) | MacGregor et al | Robinson and Fleming | Drumm et al |
| (cm) | Gestational Age | Gestational Age | Gestational Age |
| 1.0 | 7 + 5 | 7 + 0 | 6 + 6 |
| 1.1 | 7 + 6 | 7 + 1 | 7 + 1 |
| 1.2 | 8 + 0 | 7 + 3 | 7 + 2 |
| 1.3 | 8 + 1 | 7 + 4 | 7 + 3 |
| 1.4 | 8 + 1 | 7 + 5 | 7 + 4 |
| 1.5 | 8 + 2 | 7 + 6 | 7 + 5 |
| 1.6 | 8 + 3 | 8 + 0 | 7 + 6 |
| 1.7 | 8 + 4 | 8 + 1 | 8 + 0 |
| 1.8 | 8 + 5 | 8 + 2 | 8 + 1 |
| 1.9 | 8 + 5 | 8 + 3 | 8 + 2 |
| 2.0 | 8 + 6 | 8 + 4 | 8 + 3 |
| 2.1 | 9 + 0 | 8 + 5 | 8 + 4 |
| 2.2 | 9 + 1 | 8 + 6 | 8 + 5 |
| 2.3 | 9 + 1 | 8 + 6 | 8 + 6 |
| 2.4 | 9 + 2 | 9 + 0 | 9 + 0 |
| 2.5 | 9 + 3 | 9 + 1 | 9 + 1 |
| 2.6 | 9 + 4 | 9 + 2 | 9 + 2 |
| 2.7 | 9 + 4 | 9 + 3 | 9 + 3 |
| 2.8 | 9 + 5 | 9 + 3 | 9 + 3 |
| 2.9 | 9 + 6 | 9 + 4 | 9 + 4 |
| 3.0 | 9 + 6 | 9 + 5 | 9 + 5 |
| 3.1 | 10 + 0 | 9 + 6 | 9 + 6 |
| 3.2 | 10 + 1 | 9 + 6 | 10 + 0 |
| 3.3 | 10 + 2 | 10 + 0 | 10 + 0 |
| 3.4 | 10 + 2 | 10 + 1 | 10 + 1 |
| 3.5 | 10 + 3 | 10 + 1 | 10 + 2 |
| 3.6 | 10 + 4 | 10 + 2 | 10 + 3 |
| 3.7 | 10 + 4 | 10 + 3 | 10 + 3 |
| 3.8 | 10 + 5 | 10 + 3 | 10 + 4 |
| 3.9 | 10 + 6 | 10 + 4 | 10 + 5 |
| 4.0 | 10 + 6 | 10 + 5 | 10 + 5 |
| 4.1 | 11 + 0 | 10 + 5 | 10 + 6 |
| 4.2 | 11 + 1 | 10 + 6 | 11 + 0 |
| 4.3 | 11 + 1 | 11 + 0 | 11 + 0 |
| 4.4 | 11 + 2 | 11 + 0 | 11 + 1 |
| 4.5 | 11 + 3 | 11 + 1 | 11 + 2 |
| 4.6 | 11 + 3 | 11 + 1 | 11 + 2 |
| 4.7 | 11 + 4 | 11 + 2 | 11 + 3 |
| 4.8 | 11 + 5 | 11 + 3 | 11 + 4 |
| 4.9 | 11 + 5 | 11 + 3 | 11 + 4 |
| 5.0 | 11 + 6 | 11 + 4 | 11 + 5 |
| 5.1 | 12 + 0 | 11 + 4 | 11 + 5 |
| 5.2 | 12 + 0 | 11 + 5 | 11 + 6 |
| 5.3 | 12 + 1 | 11 + 5 | 12 + 0 |
| 5.4 | 12 + 1 | 11 + 6 | 12 + 0 |
| 5.5 | 12 + 2 | 11 + 6 | 12 + 1 |
| 5.6 | 12 + 3 | 12 + 0 | 12 + 2 |
| 5.7 | 12 + 3 | 12 + 1 | 12 + 2 |
| 5.8 | 12 + 4 | 12 + 1 | 12 + 3 |
| 5.9 | 12 + 4 | 12 + 2 | 12 + 3 |
| 6.0 | 12 + 5 | 12 + 2 | 12 + 4 |
| 6.1 | 12 + 6 | 12 + 3 | 12 + 5 |
| 6.2 | 12 + 6 | 12 + 3 | 12 + 5 |
| 6.3 | 13 + 0 | 12 + 4 | 12 + 6 |
| 6.4 | 13 + 0 | 12 + 4 | 12 + 6 |
| 6.5 | 13 + 1 | 12 + 5 | 13 + 0 |
| 6.6 | 13 + 2 | 12 + 5 | 13 + 0 |
Limitations of CRL measurement:
- After 13 weeks, the CRL becomes less accurate for dating the pregnancy because the embryo grows at a slower and more variable rate.
- If the image quality of the ultrasound is poor, it may be difficult to obtain an accurate CRL measurement.
Mean Sac Diameter (MSD) Calculation
The mean sac diameter (MSD) is a measurement of the gestational sac, the fluid-filled sac that surrounds the embryo in early pregnancy. The gestational sac is visible on ultrasound before the embryo itself can be seen.
How to measure MSD:
During an early pregnancy ultrasound, the healthcare professional will measure the gestational sac in three different planes. The mean sac diameter is then calculated by averaging these measurements.
How to calculate gestational age from Mean Sac Diameter?
You can do this by using formulas or charts.
Formula for calculating gestational age from MSD: Days of pregnancy = mean sac diameter (in mm) + 30
Gestational Age from MSD using Charts:
| Gestational Sac | Gestational Age | Gestational Sac | Gestational Age |
| Mean Diameter (mm) | (weeks) | Mean Diameter (mm) | (weeks) |
| 10 | 5.0 | 30 | 7.9 |
| 11 | 5.2 | 31 | 8.0 |
| 12 | 5.3 | 32 | 8.2 |
| 13 | 5.5 | 33 | 8.3 |
| 14 | 5.6 | 34 | 8.5 |
| 15 | 5.8 | 35 | 8.6 |
| 16 | 5.9 | 36 | 8.8 |
| 17 | 6.0 | 37 | 8.9 |
| 18 | 6.2 | 38 | 9.0 |
| 19 | 6.3 | 39 | 9.2 |
| 20 | 6.5 | 40 | 9.3 |
| 21 | 6.6 | 41 | 9.5 |
| 22 | 6.8 | 42 | 9.6 |
| 23 | 6.9 | 43 | 9.8 |
| 24 | 7.0 | 44 | 9.9 |
| 25 | 7.2 | 45 | 10.0 |
| 26 | 7.3 | 46 | 10.2 |
| 27 | 7.5 | 47 | 10.3 |
| 28 | 7.6 | 48 | 10.5 |
| 29 | 7.8 | 49 | 10.6 |
This chart is based on : Hellman LF, Kobayashi M, Fillisti L et al: Growth and development of the human fetus prior to the twentieth week of gestation. Am J Obstet Gynecol 103:789, 1969.
Limitations of MSD measurement:
- The MSD is only accurate for dating the pregnancy up to about 10 weeks of gestation.
- The MSD can be variable, even in healthy pregnancies.
Estimated Due Date Calculation
There are two main methods for calculating an estimated due date (EDD):
- Using the last menstrual period (LMP): This is the most common method for dating a pregnancy. If you have a regular menstrual cycle, your healthcare provider can calculate your EDD by counting 280 days from the first day of your last menstrual period. However, this method is not as accurate if you have irregular periods or if you don’t remember the date of your last period.
- Using ultrasound measurements: Early pregnancy ultrasound measurements, such as CRL and MSD, can be used to estimate the EDD. These measurements are most accurate in the first trimester.
Combining LMP and ultrasound measurements:
In many cases, healthcare providers will use both the LMP and early pregnancy ultrasound measurements to calculate the EDD. This approach is usually more accurate than using either method alone.
While the CRL and mean sac diameter measurements provide valuable information, they are not the only factors considered when assessing gestational age. Healthcare providers will also take into account the date of the last menstrual period, any previous ultrasound findings, and other relevant medical history to provide the most accurate assessment possible.
Importance of Accurate Gestational Age Determination
Accurate gestational age determination is essential for scheduling important prenatal tests and screenings at the appropriate times during the pregnancy. It helps healthcare providers monitor the baby’s growth and development, identify any potential issues or complications early on, and plan appropriate prenatal care and interventions if necessary.
If you’re pregnant or planning to become pregnant, it’s essential to understand the importance of these measurements and the role they play in ensuring a healthy pregnancy. Regular prenatal care and open communication with your healthcare provider are key to monitoring your baby’s progress and addressing any concerns that may arise.
References from peer reviewed journals:
- Tan, C., Ellewela, C. N., Sandhu, S. S., Melamed, N., Librach, C. L., & Aviram, A. (2023). A new formula for estimating gestational age by crown-rump length. American Journal of Obstetrics & Gynecology MFM, 6(2), 101254.
- Robinson, H.P. and Fleming, J.E.E. (1975), A CRITICAL EVALUATION OF SONAR “CROWN-RUMP LENGTH” MEASUREMENTS. BJOG: An International Journal of Obstetrics & Gynaecology, 82: 702-710.
- MacGregor, S, Sabbagha, R, Glob. libr. women’s med., (ISSN: 1756-2228) 2008;
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Disclaimer: The author makes no claims of the accuracy of the information contained herein; this information is for educational purposes only and is not a substitute for clinical judgment.