12 questions on pregnancy test kits: How do they work? Can they tell if I've had a miscarriage?

When should a woman use a pregnancy test kit to check if she’s pregnant? Does the price and type matter? And can a urine test kit reveal a recent miscarriage? Obstetricians and gynaecologists Dr Serena Koh and Dr Heng Tung Lan answer your questions on pregnancy sticks.

Have you missed your period? Do you feel a bit nauseous? Perhaps you’ve noticed some bloating? Many factors could explain what you’re experiencing. Your period could simply be late, or... you might be pregnant.

To know for sure, you can buy an over-the-counter urine pregnancy test kit from the pharmacy. All you need to do is pee on the stick, wait for the duration recommended on the package, and the answer will be clear.

Or will it? 

How exactly does a pregnancy test kit work and is it accurate every single time? Can a pregnancy test kit detect when you’ve had a miscarriage?

Dr Serena Koh from KK Women’s and Children’s Hospital and Dr Heng Tung Lan from Parkway East Hospital, both obstetricians and gynaecologists, shed light on pregnancy test kits. 

1. HOW DO PREGNANCY TEST KITS WORK?

During pregnancy, a woman’s placenta produces a hormone called human chorionic gonadotropin (hCG) that circulates in the blood and is present in her urine. The pregnancy test kit detects the presence of this hormone in the urine, said Dr Koh.

2. HOW ACCURATE IS A PREGNANCY TEST KIT?

Dr Koh said that over-the-counter urine pregnancy test kits are accurate more than 95 per cent of the time.

In some cases, a test may show a negative result even though the woman is pregnant. Dr Heng said this happens when hCG levels are low, which is common during early pregnancy, typically six to 12 weeks after the date of the last missed period.

3. DOES IT MATTER WHAT BRAND OR TYPE OF KIT IS USED? ARE DIGITAL OR MORE EXPENSIVE KITS MORE ACCURATE?

In terms of accuracy, both doctors say there isn’t really a difference. 

Digital test kits display the result as “Pregnant” or “Not Pregnant”,  and some may even indicate the number of weeks pregnant. In contrast, non-digital test kits give you a line that indicates a positive result. That line may require interpretation as it may be faint and take time to form.

While digital test kits may remove the need for interpretation, they aren’t necessarily more accurate compared with their non-digital counterparts, said Dr Heng.

Certain test kits, typically the pricier ones, may be more sensitive to hCG levels. This sensitivity could yield an accurate result sooner or even before your expected period. However, this isn’t guaranteed as it still depends on your body’s hCG level. 

Regardless of type, sensitivity and cost, Dr Koh said most pregnancy test kits should be able to detect hCG in the urine about two to three weeks after conception.

Given that most brands are reliable and there aren’t significant differences in accuracy, a woman should select a test kit she’s comfortable with and can afford. Dr Koh also advised carefully reading and following the instructions for the test kit, as usage instructions may vary.

4. HOW MANY DAYS AFTER HAVING SEX SHOULD A WOMAN TAKE THE TEST?

A pregnancy test can be positive around a week or two after having sex, Dr Heng said. But to be sure, women are advised to take the test once they’ve missed their period. For women with regular menstrual cycles, the test should show a positive result once they’ve missed their period by a few days.

If a woman has problems calculating her ovulation period, Dr Koh advises she download a reliable mobile application to help her.

5. SHOULD WOMEN WITH IRREGULAR PERIODS WAIT LONGER BEFORE TAKING A TEST?

If periods are very irregular, it may be difficult to time when to take a pregnancy test or one may need to take it multiple times to be sure, said Dr Heng. 

In such cases, she suggested waiting for early symptoms of pregnancy, such as nausea, breast tenderness, or bloating, before taking a test.

6. IS THERE A BEST TIME – DURING THE DAY OR NIGHT – TO TAKE A PREGNANCY TEST?

Women can take the pregnancy test at any time of the day, Dr Heng said. 

However, the first-morning urine specimen is typically the most concentrated of the day, containing the highest levels of hCG. This, Dr Koh explained, makes it more accurate for pregnancy testing.

7. DOES IT MATTER HOW DARK OR FAINT THE LINES ARE WHEN TESTING FOR PREGNANCY?

The darkness or faintness of the lines on a pregnancy test kit indicates the level of hCG detected in the urine, Dr Koh said. 

A darker line indicates a higher concentration of hCG, often associated with a more advanced stage of pregnancy. A faint line can still indicate a positive result, as long as it appears within the specified time frame outlined in the test instructions. Hence, it is important to follow the manufacturer’s guidelines. 

If you have doubts, Dr Koh said to consider repeating the test using a fresh stick, either from the same brand, or a different brand or type of kit, or consult a doctor for confirmation.

8. AFTER A POSITIVE PREGNANCY RESULT ON A TEST KIT, SOME WOMEN MAY GET A NEGATIVE ON A BLOOD TEST – HOW MIGHT THIS HAPPEN?

A negative pregnancy blood test after a positive urine test often means that the level of hCG is decreasing and the initial pregnancy is not progressing well, Dr Heng said.

There are also rare examples when the hCG level is high for a short period and then drops to zero, Dr Koh added. This is known as a biochemical pregnancy, which refers to a very early miscarriage that occurs in the first few days of pregnancy.

The embryo produces sufficient amounts of hCG for the pregnancy hormone to be detected on the initial pregnancy test, but does not progress into a clinical pregnancy.

This is not a false positive, Dr Koh said, as the woman is truly pregnant, even if the pregnancy cannot be seen on an ultrasound scan and may not be viable.

In such cases, the initial rise of hCG will soon drop and result in a negative urine pregnancy test. In a biochemical pregnancy, vaginal bleeding will also typically occur soon afterwards, which will coincide with delayed menses. Hence, some delayed periods may actually be a result of an early miscarriage. 

9. CAN OVER-THE-COUNTER TEST KITS REVEAL IF A WOMAN HAD AN EARLY MISCARRIAGE?

Dr Heng said that after a miscarriage or pregnancy termination, a pregnancy test kit may still come up as positive for some time because the hCG level can remain high even if the foetus is no longer developing in the womb.

As false positives can occur, to tell for sure, Dr Koh recommends getting a blood test at any clinic, including your general practitioner or polyclinic. Blood tests are more accurate as they measure actual hormone levels, whereas urine tests merely tell you if hCG is present.  

A blood test can also help differentiate between a miscarriage, where blood hCG decreases over time, and a healthy pregnancy, where the blood hCG level typically doubles every two days.

10. WHAT CAN CAUSE A FALSE POSITIVE RESULT ON A PREGNANCY TEST KIT? 

If a pregnancy test is left for a few hours after taking it, it may show a false positive, which is why Dr Koh advises women to read the results within five to 10 minutes.

False positives may also show up if the pregnancy test is done soon after taking fertility medicines that contain beta hCG hormones, as part of an assisted reproductive technology cycle, she added. 

In rare instances, ovarian tumours can secrete hCG in a woman’s body, resulting in a false positive.

As a few factors can result in a false positive, you can take the test over a few days or visit the clinic to get a blood test. 

In rare cases, a woman in her late second or third trimester of pregnancy may have a negative urine pregnancy test. 

Dr Koh explained that this is due to the hook effect, which arises due to the very high levels of hCG in the woman’s body. The antigens that urine pregnancy tests use are unable to detect the hCG hormone, giving a negative result.   

11. WHAT HAPPENS IF A WOMAN USES AN EXPIRED PREGNANCY TEST KIT?

If the urine pregnancy test kit is expired, Dr Koh said that it is no longer accurate as the antibodies used to detect hCG levels begin to break down. 

Consequently, it may show a false positive or false negative result, and such a result cannot be interpreted with any accuracy.

12. DO SEXUAL OR GYNAECOLOGICAL ISSUES SUCH AS SEXUALLY TRANSMITTED INFECTIONS, ENDOMETRIOSIS, POLYCYSTIC OVARY SYNDROME AFFECT THE RESULT  OF A PREGNANCY TEST?

Sexual and gynaecological conditions don’t affect the results directly, although they can make it tricky to time when to take a pregnancy test, Dr Koh said.

This is because certain medications or treatments used to manage these conditions may affect the accuracy of pregnancy tests, she explained.

For example, polycystic ovary syndrome (PCOS) can cause a hormonal imbalance that might affect ovulation and menstrual cycles, potentially leading to irregular periods or difficulty in predicting ovulation.