Gestational age is the medical standard for measuring how far along a pregnancy has progressed, calculated in weeks from the first day of a woman's last menstrual period (LMP) to the current date. When LMP is unknown, a sonographer can determine gestational age via ultrasound by measuring either the diameter of the gestational sac or the crown-rump length (CRL) of the fetal pole — for example, a gestational sac measuring 5mm yields an estimate of 4 weeks and 5 days when added to a baseline of 28 days. A pregnancy wheel, also called a gestation calculator, offers a quick manual alternative, using the LMP to project an estimated due date by rotating a circular calendar dial. Together, these tools — ultrasound biometry, LMP dating, and the pregnancy wheel — give clinicians and expectant parents a reliable framework for tracking fetal development throughout pregnancy.
- Gestational age is calculated in weeks, measured from the first day of the last menstrual cycle to the current date or appointment day.
- During an ultrasound scan, a sonographer calculates gestational age by measuring the diameter of the gestational sac or the Crown-Rump Length (CRL) of the fetal pole.
- A gestational sac measuring 5mm, when added to the baseline of 28 days, yields a gestational age of 33 days, or 4 weeks and 5 days.
- The formula used adds the millimetre measurement of the gestational sac to 4 weeks (28 days) to produce a general estimate of gestational age.
- A pregnancy wheel, also called a gestation calculator, uses the Last Menstrual Period (LMP) to calculate an estimated due date by rotating a pointer to the LMP date.
- Early pregnancy scans at Merrion Fetal Health are offered from 6 to 11 weeks, while the Anatomy Survey scan is performed between 18 and 22 weeks.
- Post Dates and Liquor scans at Merrion Fetal Health are available from 40 weeks onwards.
What is gestational age and how to measure it?
If you are pregnant and need to calculate the gestational age of your fetus and understand what is gestational age, then read on.
What is gestational age?
Once you start attending a medical practitioner and going for early pregnancy scans, you will hear the term gestational age. If it is your first pregnancy you may not be familiar with what this means.
Simply it is the term used medically to calculate how far along your pregnancy is. It is calculated in weeks and measured from the first day of your last menstrual cycle to today’s date or the day of your appointment.
How ultrasound scans can measure gestational age
Of course, not all of us keep track of our first day of menstruation religiously. So what happens in these common cases? When you come for an ultrasound pregnancy scan, the sonographer will be able to tell the gestational age of your fetus.
This is calculated by measuring the diameter of the gestational sac or CRL – the length of the fetal pole. If you’re curious about how we do this during your ultrasound scan, here are some example of the formula:
We take the mm measurements of your fetus’ gestational sac and add this to 4 weeks/28 days. This gives you a general estimate of gestational age. So for example, if the sac measures 5mm, we add this to 28 and get a total of 33 days, which would be 4 weeks and 5 days in total. Also, charts have been developed to illustrate and calculate this.
What is the pregnancy wheel?
A pregnancy wheel is a small circular calendar that is also referred to a gestation calculator. By using your LMP – last menstrual period, the pregnancy wheel can calculate your estimated due date. Simple move its pointer to your LMP rotating the wheel to show the estimated due date.
Image credit for pregnancy wheel FaceMama
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All articles on the blog and website are intended as information only. Please do not consider any of the information provided here as a substitute for medical advice. At all times seek medical advice directly with your own doctor and medical team.
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This website was formerly Merrion Fetal Health. The clinic has undergone a rebrand and is now known as Merrion Ultrasound.
What is gestational age?
Gestational age is the medical term used to calculate how far along a pregnancy is. It is measured in weeks, calculated from the first day of the last menstrual cycle to the current date or the date of a medical appointment.
How is gestational age measured if you don't know your last period date?
A sonographer can determine gestational age during an ultrasound scan by measuring the diameter of the gestational sac or the crown-rump length (CRL), which is the length of the fetal pole. These measurements are then used in a formula to estimate how many weeks pregnant you are.
How does an ultrasound calculate gestational age from the gestational sac?
The sonographer takes the millimetre measurement of the gestational sac and adds it to 4 weeks (28 days) to produce a general estimate of gestational age. For example, if the sac measures 5mm, adding 28 days gives a total of 33 days, or 4 weeks and 5 days.
What is a pregnancy wheel?
A pregnancy wheel, also called a gestation calculator, is a small circular calendar used to calculate an estimated due date. By rotating the wheel's pointer to align with the date of the last menstrual period (LMP), it displays the estimated due date.
What does CRL stand for in pregnancy scans?
CRL stands for crown-rump length, which is the measurement of the length of the fetal pole. Sonographers use this measurement during early ultrasound scans to help calculate gestational age.
How is gestational age calculated from the last menstrual period?
Gestational age is calculated by counting the number of weeks from the first day of the last menstrual period (LMP) to the current date. This method is used by medical practitioners and tools such as the pregnancy wheel.
Which measurements do sonographers use to determine gestational age on ultrasound?
Sonographers use two key measurements: the diameter of the gestational sac and the crown-rump length (CRL) of the fetal pole. These measurements are applied to established formulas and charts to estimate how far along the pregnancy is.
What scans are available during pregnancy and at which weeks?
Common pregnancy scans include early pregnancy scans at 6–11 weeks, nuchal translucency scans at 12–14 weeks, anatomy surveys at 18–22 weeks, and gender scans from 19 weeks onwards. Growth and well-being scans are available from 22 weeks, and post-dates scans from 40 weeks onwards.