# Calculating gestational age of a baby

Does "gestational age" of an unborn child count from the first day of the last menstrual period of the mother?

In the absence of reasons to use another date, yes, the first day of the last menstrual period is used.

There are cases where this is inaccurate, for example women with cycles that deviate from 28 days by more than a few days, or women who got pregnant using a birth control method that suppresses periods (implant, continuous birth control pills, some IUDs). In those cases, the earliest ultrasound in which an embryo can be seen (usually week 6 to 10 of pregnancy) is used.

How Doctors Date Pregnancies

• I don't think that's accurate for the unusual cases. They estimate the age of the embryo from the size at first ultrasound, and backdate what would have been the first day of the last menstrual period if it had occurred. Oct 15, 2016 at 19:00
• @Warren how is that different from what I wrote in my second paragraph? Oct 16, 2016 at 9:42
• Your second paragraph implies that the pregnancy is dated from the date of the ultrasound, rather than backdated based on the embryo size at the ultrasound. Perhaps an edit for clarification is in order. Oct 16, 2016 at 18:19

Yes, the gestational age is counted from the first day of the mother's most recent menstrual cycle. From Gestational Age on the NIH site:

Gestational age is the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks, from the first day of the woman's last menstrual cycle to the current date. A normal pregnancy can range from 38 to 42 weeks.

• So now the baby gestational age is 24 weeks and delivery date is 23 feb 2016. And if i counting back for last menstrual periods. Its 20 may. And we made last intercource on 7th may. Could i think that this child is not mine ??????? Plz reply Nov 2, 2015 at 11:40
• @Ali you are not going to get a definitive answer about paternity from strangers on the internet; calculating date of conception from gestational age or last period are not perfectly accurate. Nov 2, 2015 at 11:53

Gestational age is the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks, from the first day of the woman's last menstrual cycle to the current date. A normal pregnancy can range from 38 to 42 weeks.

Select the first day of the last menstrual period and the calculator considers all for you http://calcsoft.com/pregnancy-calculator

Gestational age, or the age of the baby, is calculated from the first day of the mother’s last menstrual period. Since the exact date of conception is almost never known, the first day of the last menstrual period is used to measure how old the baby is. i suggest you to take a look at this tool https://calculatricedegrossesse.com/

• Hi yola, and welcome. Link-only answers are subject to deletion on SE sites, as they are subject to link rot (and also probably the reason for the downvote.) Editing your answer to include pertinent information would be welcome and more helpful; if you don't care to do that, the answer will be removed eventually. Thanks, and again, welcome. Oct 14, 2016 at 3:02
• how about now i have tried to add value to my answer as suggeted thank you guy for the tips
– yola
Oct 14, 2016 at 12:38
• Thank you! I have upvoted. I hope others will see the effort you made and will do the same, or at least remove the downvote. :) Oct 14, 2016 at 12:52
• The first day of the last menstrual period is used even if the exact date of conception is known precisely, such as in IVF. Oct 15, 2016 at 18:59