A very common question for many women, pregnant or not, is how to count the weeks of pregnancy, even when having regular cycles1. Another question is whether you can calculate pregnancy duration only by the levels of HCG hormone, which is detected by blood tests. Counting the weeks of pregnancy is not all that complicated. But we can also say it’s not so easy to understand. That’s why an explanation on how to count the weeks of pregnancy as well as the estimated due date can become quite a journey and cause confusion.

Counting from the Day of Menstruation

Pregnancy lasts 280 days or 40 weeks, right? So we count the weeks of pregnancy starting from the first day of the menstruation. It may sound strange, I know, but this is how doctors know exactly how to count the weeks of pregnancy so the baby is born at the appropriate time. So, for the calculation, the doctor will ask: When was the first day of your last period?

But it’s not like this for all women. In some cases, counting the weeks of pregnancy by the menstruation might be completely incorrect, whether due to a delay or simply because the woman doesn’t remember the day she menstruated—or even due to errors in calculations, like having bled while already pregnant.

IMPORTANT: If during your pregnancy the ultrasound shows your baby is larger or heavier than estimated in your prenatal calculations, don’t panic. The ultrasound is an estimate and the measurements may vary.

When Does HCG Appear in the Blood?

HCG starts to appear in a woman’s body after implantation, and then both pharmacy and blood tests will start turning positive for quantities above 25mUI2. In some labs, only above 50MUIml is considered positive. However, very early in pregnancy, the hormone can still be quite low. Around 20, 50, 100mUI. These amounts might seem low, but it’s normal if you test too early.

There are some HCG level tables on the internet, but none of them can give a completely accurate answer to the well-known question of how many weeks pregnant am I. They can only provide an estimate. That’s because every woman’s body might secrete more or less HCG. Detection can vary from woman to woman, also because it can depend on hormonal variation and the number of babies.

Is the HCG Amount Higher with Twins?

So if you’re pregnant with twins, the tendency is that HCG levels will be higher, but this isn’t always the case. Reference tables from laboratories can also vary greatly and may confuse or even worry you since they might be higher or lower than your test result3. Therefore, determining how to count the weeks of pregnancy by HCG level is not the best method. The date of the last period makes all the difference. But what about those who don’t remember when their last period was?

Weeks of Pregnancy – How to Get the Count Exactly Right?

For those who want to know how many weeks pregnant they are and don’t remember the date of their last period, an ultrasound is needed. The doctor may request the first ultrasound, which can determine with accuracy the gestational age, the stage, and the weeks of pregnancy. The earlier, the better for finding out. But starting from the 5th week of pregnancy, it’s possible to have an accurate calculation of how many weeks pregnant you are and the baby’s estimated due date.

All this is because the baby’s size and weight can start to differ after a while, so the further along the pregnancy (the bigger the baby), the higher the chances of errors or a big difference in the estimated birth date. Incorrect measurements can lead to mistakes about the EDD (estimated due date).

How to Confirm?

In summary: if you have regular cycles and your menstruation is predictable, the date of your last period works well to find out how many weeks pregnant you are. But if your cycles are irregular or you don’t remember your last period, then ultrasound is the best way to count the weeks of pregnancy. It is also important to know that ultrasounds done later in pregnancy can show variation in the estimated birth date. So, you should really consider the date of the first ultrasound.

Readers’ Questions:

Can an ultrasound show the weeks of pregnancy?

Yes, an ultrasound is recommended early in pregnancy to confirm the gestational age, as well as to see if the development is on track. As pregnancy progresses, ultrasound should not be used to determine gestational age since the baby’s growth and weight gain can vary from pregnancy to pregnancy.

See also: Dads Receiving the Pregnancy News

Photo: Trevor Bair