How do pregnancy tests work? - Tien Nguyen

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Over-the-counter pregnancy tests give potentially life-changing results with a pretty high rate of accuracy. But how do they work? Tien Nguyen explains how each test performs a scientifically rigorous, multi-stage experiment that goes from start to finish in the time that it’ll take you to watch this video.

Lesson by Tien Nguyen, animation by Andrew Foerster.

The earliest known pregnancy test
dates back to 1350 BC in Ancient Egypt.

According to the Egyptians,

all you have to do is urinate on wheat
and barley seeds, and wait.

If either sprouts,
congratulations, you're pregnant!

And if wheat sprouts faster, it's a girl,
but if barley, it's a boy.

In 1963, a small study
reproduced this test

and found that it predicted pregnancy
with a respectable 70% accuracy,

though it couldn't reliably
tell the sex of the baby.

Scientists hypothesized
that the test worked

because pregnant women's urine
contains more estrogen,

which can promote seed growth.

Now it's easy to take
this ancient method for granted

because modern pregnancy tests give
highly accurate results within minutes.

So how do they work?

Over-the-counter pregnancy tests
are all designed to detect one thing:

a hormone called HCG.

HCG is produced in
the earliest stages of pregnancy

and starts a game of telephone

that tells the body not to shed
the inner lining of the uterus that month.

As the pregnancy progresses,

HCG supports the formation
of the placenta,

which transfers nutrients
from mother to fetus.

The test starts when urine is applied
to the exposed end of the strip.

As the fluid travels up
the absorbent fibers,

it will cross three separate zones,
each with an important task.

When the wave hits the first zone,
the reaction zone,

Y-shaped proteins called antibodies
will grab onto any HCG.

Attached to these antibodies
is a handy enzyme

with the ability to turn on dye molecules,
which will be crucial later down the road.

Then the urine picks up
all the AB1 enzymes

and carries them to the test zone,
which is where the results show up.

Secured to this zone are more
Y-shaped antibodies

that will also stick to HCG
on one of its five binding sites.

Scientists call this type of test
a sandwich assay.

If HCG is present, it gets sandwiched
between the AB1 enzyme and AB2,

and sticks to the test zone,

allowing the attached dye-activating
enzyme to do its job

and create a visible pattern.

If there's no HCG, the wave of urine
and enzymes just passes on by.

Finally, there's one last stop to make,
the control zone.

As in any good experiment,

this step confirms that
the test is working properly.

Whether the AB1 enzymes never saw HCG,

or they're extras because Zone 1
is overstocked with them,

all the unbound AB1 enzymes picked up
in Zone 1 should end up here

and activate more dye.

So if no pattern appears,
that indicates that the test was faulty.

These tests are pretty reliable,
but they're not failproof.

For instance, false negatives can occur

if concentrations of HCG
aren't high enough for detection.

After implantation, HCG levels double
every two to three days,

so it may just be too early to tell.

And beverages can dilute the urine sample,

which is why doctors recommend taking
the test first thing in the morning.

On the other hand, false positives
can come from other sources of HCG,

like IVF injections, ectopic pregnancies,

or certain cancers such as uterine cancer
or testicular cancer,

making it possible for one of these tests
to tell a man he's pregnant.

The best way for a woman to find out
for sure is at the doctor's office.

The doctors are also looking for HCG,

but with tests that are more sensitive
and quantitative,

which means they can determine
the exact level of HCG in your blood.

A few minutes can feel like forever

when you're waiting on the results
of a pregnancy test.

But in that brief time, you're witnessing
the power of the scientific method.

That one little stick
lets you ask a question,

perform a controlled experiment,

and then analyze the results
to check your original hypothesis.

And the best part is you won't even
have to wait until the next harvest.