Tracking a Pregnancy Using Gestational Age

Ultrasound is the Best Way to Determine or Confirm Gestational Age

Doctor and pregnant woman looking at digital tablet
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Gestation is a term that describes the time between conception and birth during which a baby grows and develops in the mother's uterus. Because so many important milestones take place during this period to indicate a pregnancy is progressing normally, it is common for women to track their pregnancies to ensure these milestones are hit at the appropriate time.

Why Gestational Age Is Important

Gestational age is the usual way for describing the age of the pregnancy, or how far along it is. Usually expressed as a combination of weeks and days, gestational age counts from the first day of the mother's last menstrual period to the present, so it technically includes about two weeks during which the woman was not pregnant.

Gestational age helps guide prenatal care. In addition, it yields an expected due date and is the method most physicians use for dating a pregnancy. Gestational age is different from fetal age, which is the number of weeks that have passed since conception. 

Most pregnancies will last about 40 weeks when using gestational age to estimate the due date, but anything from 38 weeks to 42 weeks is considered normal. Babies born before 37 weeks are considered premature and those born after 42 weeks considered postmature.

Why Doctors Use Gestational Age to Date a Pregnancy

The reason gestational age is used so often in clinical practice is because it is rare in most pregnancies to know exactly when conception occurred.

However, in some pregnancies that is not the case. For example, women who became pregnant with the assistance of fertility treatments such as in-vitro fertilization (IVF) or intrauterine insemination may know exactly when the pregnancy began and this information may be used instead of gestational age to date the pregnancy.

In other cases, a woman may believe she knows exactly when she conceived based on the timing and frequency of intercourse and the features and experience of her menstrual cycle. 

Drawbacks of Using Gestational Age

It is important to keep in mind that gestational age calculations assume a 28-day menstrual cycle for all pregnant women, in which ovulation takes place on day 14. In reality, many menstrual cycles are significantly shorter or longer. Especially in cases of irregular cycles, gestational age should be used with caution as it may over- or under-estimate the true age of the developing embryo or fetus. 

This can have implications for prenatal testing and diagnosis. For example, if an ultrasound performed at 7 weeks gestation shows development that is normal for 6 weeks gestational age, this could raise concern for the woman and her doctor. However, if that woman experienced a 35-day menstrual cycle in the month she became pregnant, these results would be much less alarming. 

A Word From Verywell

Because of the above example, which happens regularly in clinical practice, ultrasound is considered the gold standard for estimating or confirming gestational age. In the first trimester, the gestational age is determined by measuring the crown-rump length of the fetus via ultrasound. Because this measurement is so accurate, the gestational age calculated by your last period may be corrected if it is off from the ultrasound measurement by five to seven days. 

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