Navigating Pregnancy with Irregular Menstrual Cycles: How to Calculate Gestational Age

Gestational age is an important part of understanding pregnancy, particularly when it comes to abortion. It’s essential to know the gestational age as it will impact what abortion methods are suitable as well as the safety and legality of the abortion itself.

Gestational age estimation is usually derived from your last menstrual period (LMP) and is fairly easy to calculate for those with regular periods. However, an issue arises for women who have irregular periods because ovulation may not happen consistently at the same point in their cycle each month.

So, how do you calculate gestational age when you have irregular periods? This article will discuss the different methods for estimating gestational age when you can’t necessarily rely on your LMP.

Methods to estimate gestational age without an LMP

There are a few ways that providers can make an educated guess about your gestational age when you don’t have an LMP, including using ultrasound scans and physical exams.

Ultrasound scan

You’ve probably heard of an ultrasound scan, or perhaps you’ve received one in the past. Ultrasounds utilize high-frequency sound waves to generate images of what’s happening inside the body. This is particularly helpful during pregnancy as it allows us to see the developing fetus, thus allowing for assessment of how the pregnancy is progressing.

Ultrasound scans can also be very helpful when estimating gestational age. Using an ultrasound to measure the embryo or fetus during the first trimester is the most effective way to confirm gestational age (1). Thus, if you have a confirmed pregnancy, an ultrasound is one of the first things your provider will do.

There are two types of ultrasound procedures: transabdominal and transvaginal. With a transabdominal ultrasound, a technician applies a lubricating gel to the abdomen and uses a handheld transducer device on the skin. This transducer captures images of the fetus, uterus, and other internal structures. With a transvaginal ultrasound, the transducer is instead placed inside the vagina and similarly projects images of what’s happening inside the body (2).

Blood test

A hormone known as HCG is present in the blood of pregnant women within 10 days of conception. A quantitative HCG test measures exactly how much HCG is currently in the blood, helping to figure out gestational age. For example, an HCG value of 152 to 32,177 mIU/mL correlates to being six months along.

There are only a few cons to blood tests: feeling faint, bleeding, or infection (rare). Otherwise, these tests are fairly accurate and safe (3).

Physical exam

Your provider can also assess your physical state to further understand your gestational age. However, even though this is an important part of prenatal care, it isn’t as accurate as ultrasounds for assessing gestational age.

During your physical exam, your provider may check your height and weight to determine how much weight you should expect to gain during your pregnancy. They may also look at things like the size and position of the uterus as well as the fundal height. The fundal height is defined as the distance from the pubic bone to the top of the uterus. Usually, the fundal height correlates with how far the pregnancy has progressed, helping to determine gestational age.

Alternative calculations for irregular cycles

While ultrasounds and physical exams can be done in-office to determine gestational age, there are a few things that can also be done at home, without an LMP. These include monitoring ovulation and signs and symptoms of pregnancy.

Monitoring ovulation

For those with irregular cycles, understanding when you ovulate can help inform gestational age. Ovulation is a key part of a woman’s menstrual cycle, which includes four main phases: menstruation, the follicular phase, ovulation, and the luteal phase. But why is ovulation so important?

Ovulation occurs in the middle of your menstrual cycle, typically between days 12 and 16. During this time, luteinizing hormone (LH) increases, which initiates ovulation. During ovulation, the ovaries release an egg into the fallopian tube. It is during this time that you are most fertile as the egg is most susceptible to fertilization by sperm. Upon fertilization, the embryo implants into the uterine lining, six to ten days after ovulation.

Because ovulation is key to conception, knowing when you ovulate or when you were ovulating previously can help determine your gestational age. If you know the last time you were ovulating before getting pregnant, it can give you a general idea of your gestational age.

There are a few ways to monitor ovulation, including by tracking basal body temperature and cervical mucus.

Basal body temperature

The basal body temperature method, or BBT, involves taking your temperature every morning as soon as you wake up. After doing this for some time, you will notice that you’ll likely have a slight temperature increase on certain days. This temperature increase, which is usually only about four-tenths of one-degree Fahrenheit or two-tenths of one-degree Celsius increase, signals that ovulation has happened (4).

Cervical mucus

Cervical mucus, also known as discharge, is another way to track ovulation. You’ve probably noticed that your discharge changes in color and consistency throughout the month and that’s because you are moving through the different phases of your menstrual cycle. Your cervical mucus can range from white, clear, or yellow to sticky, slippery, or tacky. This consistency gives you clues as to where you’re at in your cycle.

What we really want to know, though, is when you are ovulating. Right before and during ovulation, your body typically produces the most mucus. During this time, the discharge will be slippery and clear, similar to raw egg whites. You can stretch the discharge between your fingers because of the texture.

If you track your cervical mucus throughout the month, you can likely pinpoint the general timeframe in which you were ovulating. This can be helpful when it comes to determining your gestational age.

Using symptoms and signs

Another method to calculate gestational age, though less reliable, is through monitoring signs and symptoms of pregnancy. Symptoms of early pregnancy, like fatigue, morning sickness, and breast tenderness, can offer insight into how far along you might be.

For example, morning sickness usually begins around week six of pregnancy, but breast changes may start as early as the fourth week. Taking note of the timing and intensity of these symptoms, especially in women with irregular cycles, can give you a general idea of gestational age. However, it is important to note that not everyone shows these symptoms, and many people don’t feel anything at all.

Mobile apps

There are also several mobile applications available that can help you track your menstrual cycle. Keep in mind, however, that these apps are not as accurate for tracking your cycle as the basal body temperature method. Additionally, if you live in a country where abortion is restricted, the data in your app could be used against you. Digital safety and protecting your personal data is paramount, so proceed with caution if using this method.

Conclusion

If you’ve discovered you are pregnant, knowing your gestational age is important whether you are planning to continue the pregnancy or have an abortion. When it comes to abortion, gestational age is essential to determine what options you have in terms of abortion method and procedure. However, calculating your gestational age can be complicated for those who have irregular periods.
When your menstrual cycle isn’t regular, you have a few options. The most accurate method is an ultrasound, which is a routine procedure done at prenatal visits at the doctor’s office. Other assessments can provide further clues as to how far along you are, for example, physical exams, ovulation monitoring, and evaluating your signs and symptoms.

Abortion support

If you need assistance in navigating an unwanted pregnancy, reach out to our counselors at safe2choose.org. Our counselors are equipped with the information and resources needed to access safe abortion support.