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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

gestational age and birth weight chart

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?

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

Pregnancy Scans, Testimonials and Information


Nuchal Translucency

12-14 weeks


Anatomy Survey

21-22 weeks


Later Dating

10-17 weeks


Gender Scan

19 weeks onwards


Growth & Well Being

22 weeks onwards


Post Dates & Liquor

40 weeks onwards


What Our Patients Say

Gráinne Macken

This is my second visit to Merrion Fetal, I was very pleased on both occasions. Lovely quiet waiting room, appointment was on time. The 20-week scan is very detailed we enjoyed watching our baby on the large TV screen. We got some beautiful photos. The nurse was very pleasant and talked us through all the measurements and anatomy. I would highly recommend this scanning clinic.”

Áine Gibney

I had the best experience at the Merrion Fetal Health clinic for my 20-week big scan. The staff were so friendly and so nice and the lovely lady who did my ultrasound scan was amazing. She was so thoroughgoing to absolutely everything and gave me such reassurance on how my baby was growing and developing. I would recommend any Mother to be to attend here if you are looking for a comfortable, reassuring and super pleasant experience.”

Linda O'Sullivan

Highly recommend! We had an early scan due to a little scare at the start of pregnancy and then another at 12 weeks to make sure all was good again. Helen who was scanning on both days was fantastic. We felt totally relaxed and un-rushed while she took her time finding the best angle of baby to get us the clearest pictures as keepsakes all while making sure everything was perfect with baby. She reassured us throughout and I can honestly say it was the best money we ever spent getting both scans done.

Please let Helen know we are 18 weeks now and flying along Highly recommend!



About The Clinic

Merrion Fetal Health Pregnancy Scans Dublin 2

About The Consultants

Dr. Rhona Mahony

About The Team

Pregnancy scans lead sonographer at Merrion Fetal Health

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18 weeks pregnancy scan facial profile Merrion Fetal Health

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Consultant Led, Centre of Medical Excellence 


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.


This website was formerly Merrion Fetal Health. The clinic has undergone a rebrand and is now known as Merrion Ultrasound.