My ex-girlfriend is pregnant. It's mine.

[golgi body]

Originally Posted by opee

Anyway, could these be false negatives? There was one positive, and two negatives so far. Skipping two periods just doesn't seem normal. What could be going on?

What you are referring to is a test's positive and negative predicting power (probability of being pregnant/not pregnant given a positive/negative test). These two measures are related to a test's sensitivity and specificity which are not typically provided by the manufacturers of over the counter pregnancy tests. Note the following definitions:

Sensitivity = Pr(T+|D+) = Probability of testing positive given you have the disease.
Specificity = Pr(T-|D-) = Probability of testing negative given you do not have the disease.

PPV = Pr(D+|T+) = Probability of disease given you tested positive
NPV = Pr(D-|T-) = Probability of no disease given you tested negative

or, expressed in another way:

...........................sensitivity x prevalence
PPV = -------------------------------------------------------------------
..........sensitivity x prevalence + (1 - specificity) x (1 - prevalence)

...........................specificity x (1 - prevalence)
NPV = ------------------------------------------------------------------
.........(1 - sensitivity) x prevalence + specificity x (1 - prevalence)


Note that unlike sensitivity and specificity, the PPV and NPV are dependent upon the prevalence of a 'disease'. A test can have 99% sensitivity and 99% specificity, but only detect 50% of the truly positive cases (possible in rare diseases). I'll admit that it's odd to think of pregnancy as a disease and that the prevalence for 'short-duration' conditions is not as easy to comprehend (or estimate) as in chronic diseases like lung cancer, but try to stay with me.

Using Baye's theorem, we can calculate the PPV and NPV if given the sensitivity and specificity of a test, and if an estimate of the disease prevalence is available. Let's work through an example.


Ex. Suppose the sensitivity and specificity of a test is 0.95 and the prevalence of pregnancy for US teenage girls is 0.12. Using the formulas above,

PPV = 0.72 (72%)
NPV = 0.99 (99%)

However, you're actually interested in Pr(D+|T-,T-,T+), the probability of being pregnant given two negatives and one positive test.

Since each test is independent of the other, this probability becomes:

.................................Pr(T-|D+)*Pr(T-|D+)*Pr(T+|D+)*Pr(D+)
Pr(D+|T-,T-,T+)= --------------------------------------------------------------------------
..........................Pr(T-|D+)*Pr(T-|D+)*Pr(T+|D+)*Pr(D+) + Pr(T-|D-)*Pr(T-|D-)*Pr(T+|D-)*Pr(D-)

or

...........................(1- sensitivity) x (1- sensitivity) x sensitivity x prevalence
Pr(D+|T-,T-,T+)= ------------------------------------------------------------------
.........{(1- sensitivity) x (1- sensitivity) x sensitivity x prevalence} + {specificity x specificity x (1- specificity) x (1-prevalence)}


and for our made up values:
Pr(D+|T-,T-,T+)= 0.01

and similarly

Pr(D-|T-,T-,T+)= 0.99

So, under these circumstances I'd say it would be highly unlikely to be pregnant. HOWEVER, remember these are made up sens., spec., and prevalence estimates. More importantly, this assumes each test was administered properly. A somewhat old meta analysis (1998) in which investigators reviewed 16 over the counter pregnancy tests, found sensitivities to be as low as 0.52 (varied drastically among volunteer groups and actual consumer groups). Using this number for the sensitivity in our example changes things drastically (PPV=0.59, NPV=0.94, Pr(D+|T-,T-,T+)= 0.27, and Pr(D-|T-,T-,T+)= 0.73).

Here is the link to the article as well as the researchers' conclusions.

Arch Fam Med -- Diagnostic Efficiency of Home Pregnancy Test Kits: A Meta-analysis, September 1998, Bastian et al. 7 (5): 465

CONCLUSIONS
Researchers have been concerned about the differences in diagnostic test characteristics when HPT kits were used outside of a controlled laboratory setting. In the hands of experienced technicians, the HPT kits have been proven to be almost as accurate (97.4%) as professional laboratory testing. However, they are less accurate when performed by consumers. The limitation of self-testing is the ability of the users to perform the test. It is essential that HPT kits provide adequate instructions that are easy to read and understand. Our study suggests that HPT kit instructions should be reviewed to (1) make sure women understand them; (2) encourage women to wait at least 2 weeks after a missed period before performing the test; and (3) notify women of the potential for false-negative results.

Clinicians should be concerned about the diagnostic efficiency of HPT kits, given the relatively low effectiveness score when used by actual patients. When a patient calls reporting a negative result, she should be encouraged to repeat the test 1 week later if she remains amenorrheic, and to call her provider if the test result remains negative. When a patient calls reporting a positive result, she should be encouraged to schedule an appointment for her first prenatal visit to confirm that she is pregnant. Because most manufacturers do not publish the results of their trials in actual patients, we are not able to report sensitivity, specificity, and test effectiveness scores for all HPT kits currently on the market. Consumer Reports ranked these kits on the basis of the manufacturer's reported accuracy, ease of use, and cost and determined Answer to be the best value. Without more information from the manufacturers, we cannot recommend 1 specific HPT kit. Further research is needed in this area.

[guitarrob]

I'm having the same kind of issue.

My ex said that she might be preggo, because she's noticing sensitivity and nausea, etc, the whole nine. she's going to get a test today, but I'm wondering if it's mine. Here's why.

She and I last had sex in April. She just cheated on me 2 sundays ago. If she is pregnant, is there a possibility that it is this other guys and not mine?

[marina]

Originally Posted by guitarrob

I'm having the same kind of issue.

My ex said that she might be preggo, because she's noticing sensitivity and nausea, etc, the whole nine. she's going to get a test today, but I'm wondering if it's mine. Here's why.

She and I last had sex in April. She just cheated on me 2 sundays ago. If she is pregnant, is there a possibility that it is this other guys and not mine?

Who knows, but I say you go halfsies on the abortion with the other dude just in case. Costs the same as a paternity test anyway. Might as well, right?

[tuttle88]

Originally Posted by marina

halfsies on the abortion

I want that on a t-shirt.

[Alexandra]

Originally Posted by guitarrob

I'm having the same kind of issue.

My ex said that she might be preggo, because she's noticing sensitivity and nausea, etc, the whole nine. she's going to get a test today, but I'm wondering if it's mine. Here's why.

She and I last had sex in April. She just cheated on me 2 sundays ago. If she is pregnant, is there a possibility that it is this other guys and not mine?

Are you asking if a woman, who had unprotected sex with two different men could possibly have been impregnated by either of them? OF COURSE IT COULD BE HIS!!

If she is pregnant, an early ultrasound will be able to date the pregnancy to within 12 hours. Pretty easy to determine whether your sperm could possibly have been there during that window yes?

[John Galt]

Originally Posted by guitarrob

I'm having the same kind of issue.

My ex said that she might be preggo, because she's noticing sensitivity and nausea, etc, the whole nine. she's going to get a test today, but I'm wondering if it's mine. Here's why.

She and I last had sex in April. She just cheated on me 2 sundays ago. If she is pregnant, is there a possibility that it is this other guys and not mine?

(843): happy early fathers day!!!
(829): im not a father
(843): about that...

[guitarrob]

Turns out she isn't pregnant. Fuck me for being an idiot.

[MorningWood]

I vote take her to SixFlags; fun for the whole family.

Originally Posted by tuttle88

I want that on a t-shirt.

You and I both.

[Somethingdumb]

Originally Posted by dzagama

lol

MOTHERFUCKER! I was going to do the same thing. Douche.

[scruffy]

what happened in the end??