Written by Nicola Salmon, reviewed by Dr Katherine Joseph. 

Nicola Salmon (she/her) is an award-winning fertility advocate, Author of “Fat and Fertile” and Fat Positive Fertility Support Specialist. This includes a BSc in Physical Sciences from University College London and an MSc in Medical Engineering and Physics from King’s College London, Diplomas in Acupuncture and Naturopathy and being a certified Non-Diet Coach. She is passionate about supporting fat folks who feel excluded or dismissed by traditional fertility spaces, offering compassionate guidance and evidence-based support to help them navigate getting pregnant without diets, shame, or stigma. Nicola’s work challenges weight bias in the fertility industry. 

Different tools for different bodies 

It makes sense that some healthcare tools come in different sizes to suit different bodies. Blood pressure cuffs need to fit your arm. We have different needle sizes to suit different body parts. Yet we assume that pregnancy tests, a tool used by millions of people each year to make decisions about their reproductive healthcare, work the same for all bodies.

And the research disagrees.

What the research says

Pregnancy tests detect a hormone called human chorionic gonadotropin (hCG), which our bodies produce after a fertilised egg attaches to the uterine lining. Recent research has explored the impact of BMI on blood hCG levels, and what they found is something every fertility and pregnancy healthcare professional needs to know.

Most studies examining the relationship between BMI and hCG in early pregnancy show that hCG concentration tends to decrease with increasing BMI.

In one study of over 2000 pregnancies, the geometric mean hCG concentration was 117 IU/L in women with a BMI under 20 and 86 IU/L for women with a BMI of 35 or higher. (Geometric mean is a statistical measure that better accounts for the wide variation in hCG levels between individuals).  A number at this end of the scale often gets called 'low', and being told your hCG is low can be worrying.



How much difference does this actually make?

The gap between the lowest and highest BMI groups was 117 IU/L against 86 IU/L - around 26% lower. Because hCG roughly doubles every 48 hours in early pregnancy, a 26% lower concentration works out at roughly a 21-hour delay in crossing any given detection threshold. The 34.8% figure from the IVF data gives about 30 hours. So: about a day. Not a fortnight, and not a wholesale unreliability. But a day matters a great deal if you are testing early, and it matters enormously if a clinician is reading your first number against a range that was never built for your body. 

What does low mean?

hCG in early pregnancy varies a lot from person to person, and there’s no single "normal" number. In practical terms, a lower level could mean that a test doesn’t show a positive result, or a healthcare provider interprets the number as something to be cautious about. This is why a lower number can feel scary, even before anyone knows how the pregnancy is progressing.

Studies of folks undergoing fertility treatment show that whilst the hCG initially measured lower in people with higher BMI, the rate at which it then rose over 48 hours remained within the normal range and showed no significant difference across BMI categories. In fact, the researchers concluded that for people with a higher BMI, the initial number may be an unreliable guide, and that the rate of rise should be used instead. This shows that among pregnancies that succeed, higher BMI shifts hCG downward; so a "low" number is less alarming than the reference range implies.

It's worth being clear about what these studies can and can't tell us. Because they only included pregnancies that went on to a live birth, they can't tell us whether a lower starting number ever signals a problem. What they do tell us is that among pregnancies that succeeded, having a higher BMI wasn't linked to a slower rate of rise. So if your body tends to produce a lower initial number, that seems to be about how concentrated the hormone appears rather than about how well your pregnancy is progressing.

It is important to note that there is a gap in the research here. 

These studies were all carried out in people undergoing fertility treatment rather than those who conceived spontaneously. While the findings are significant, it isn't yet established whether they apply equally to people who conceived without treatment.

They also measured hCG in blood, not urine. At-home pregnancy tests detect hCG in urine once it reaches a certain threshold, so if the level in your blood is lower, it's reasonable to think the level in your urine may be lower too, making it harder for a standard test to pick up. But currently, no published research has looked at this directly. One review of home pregnancy test accuracy has noted the link between BMI and serum hCG, but the specific question of how body size affects home test results remains unstudied.

Why is hCG lower in those with higher BMI

We don’t have a biological mechanism to explain why this happens for certain, but a couple of theories have been proposed.

The first is that folks with a higher BMI have a larger blood volume. When hCG is produced and secreted into the bloodstream, the same amount of hormone is diluted in a bigger volume and therefore appears at a lower concentration. Another theory is about adipose tissue (body fat). Fat tissue isn't just storage. In everybody, it's active and releases signalling molecules that communicate with the rest of the body, and these may influence how much hCG is produced.

Regardless of the underlying biology, the point is the same: fat people deserve accurate tools and non-stigmatising care. Our bodies are still doing exactly what they need to, but the “normal” hCG levels for folks with a higher BMI are deemed low relative to smaller bodies.

Why it matters:

The results of pregnancy tests can have real-world implications in the decisions folks make about their healthcare and on their mental health. This is how:

• For someone going through IVF, a phone call from the doctor worried that their initial hCG is low and the pregnancy may not be viable is devastating. A positive result confirms that hCG is present, but what tells doctors whether a pregnancy is developing is how that level rises, usually measured by two blood tests about 48 hours apart. A low starting number, or one that rises more slowly than expected, can prompt concern that the pregnancy may not continue, which is why that worried call happens. But a lower initial hCG can be completely normal at a higher BMI. If clinicians understood this, it could change how these conversations happen and spare people those agonising few days of worry.

• For someone who does not want to be pregnant, a negative pregnancy test when they are pregnant may mean that they don’t seek out abortion care as early as they might otherwise.
  

• For someone who has a history of pregnancy loss, a false negative might mean that they don’t go to their doctor as early as they could to get additional support in early pregnancy. 

Weight stigma in fertility

These aren’t theoretical situations; they are examples of how weight stigma impacts healthcare in the day-to-day world. Weight stigma is the discrimination people face based on their body size. For pregnancy tests, it shows up in multiple ways:

Exclusion from research

Reference ranges for pregnancy tests were likely developed without including diverse body types, so what is considered a typical response does not represent all bodies.

Information gaps

Most healthcare providers don’t know that blood hCG levels tend to be lower in fat folks, or what that means for at-home pregnancy tests. This means that the information and support provided in early pregnancy is not based on what might actually be going on for someone in a bigger body.

Medical assumptions

Healthcare providers use pregnancy test data to make decisions about patients’ reproductive healthcare. If that data does not reflect the nuance of different weights, it may affect the care they receive and the decisions they make.

What we can do about it:

You can’t change how your body responds, but you can choose the tools you use. 

Early detection pregnancy tests

A typical urine pregnancy test has an hCG threshold of 20 or 25 mIU/mL, whereas an early detection test, such as OVUM, has a lower threshold of 10mIU/mL. This means they can detect potentially lower concentrations that may be present due to BMI, giving you a more accurate result when you need it.

Since your hCG concentration may naturally be lower, using the most concentrated sample you can will give you the best result. Two things make the biggest difference here: testing with first morning urine, which is usually the most concentrated of the day, and not drinking a lot of fluids beforehand.

Using the research to inform your decisions

Knowledge is power. If you know that a normal pregnancy test might not be as accurate initially, you have options. You can use an early detection test or, test with a standard test and repeat it 2-3 days later if you need to. A blood test from your doctor will give you an actual hCG result. It’s OK to ask for a blood test and talk to them about this research and how it might affect your test results and decision-making.

Remember, we are not all the same. Our bodies respond in different ways, and that’s completely normal. The research highlights a trend, but this may not be true for you.

Folks in all bodies deserve to have accurate pregnancy tests when they need them. Knowing that your hCG may be lower and that it doesn’t necessarily mean anything is wrong is vital information for folks with bigger bodies and the healthcare providers supporting them.

Key Takeaways

• Pregnancy tests don't work the same way for everybody. Research shows that hCG, the hormone tests detect, tends to be lower in people with a higher BMI, and that's a normal variation, not a sign anything is wrong.

• This matters because a lower or delayed result can affect the healthcare decisions we make, from when we seek support in early pregnancy to when we seek abortion care.

• Early Detection pregnancy tests, which have a lower hCG threshold, can give you a more reliable result, and you deserve accurate information to make informed choices about your reproductive healthcare.