Gestational age
Gestational age is the standard way to say how far along a pregnancy is. It is counted from the first day of the last menstrual period, so it runs about two weeks ahead of the actual age of the embryo.
It is written as completed weeks and days, for example 14 weeks and 1 day. A full pregnancy lasts about 40 weeks.
How it is counted
The count starts on the first day of your last period, not on the day you conceived. Conception usually happens about two weeks later, so a pregnancy is already counted as two weeks along when the egg is fertilized.
If your last period began on 1 January, then on 29 January you are 4 weeks and 0 days along, and on 5 February you are 5 weeks and 0 days.
Why doctors count this way
The first day of the last period is usually the one date a woman can remember, while the day of conception is rarely known. Counting from it gives everyone the same scale, so check-ups, tests and the due date can be planned in the same way everywhere.
Gestational age and the trimesters
The trimesters follow gestational age. These are the boundaries our calculator uses, and some sources set them slightly differently.
- First trimester: weeks 1 to 13
- Second trimester: weeks 14 to 27
- Third trimester: from week 28 until birth
When the ultrasound changes it
An ultrasound early in pregnancy measures the baby and can estimate the gestational age. If the scan and the date from your last period differ noticeably, your doctor may adjust the due date. Once your doctor gives you a date, follow it.
After birth: the baby's age
After birth the baby's age is counted from the date of birth. For a baby born before 37 weeks, doctors often use a corrected age, counted from the original due date. Our baby age calculator can work it out.
This is general information, not a substitute for a doctor's advice.
Sources
- Methods for Estimating the Due DateACOG (American College of Obstetricians and Gynecologists)
- How Your Fetus Grows During PregnancyACOG (American College of Obstetricians and Gynecologists)