positive and negative urine test

So my wife has had a positive pregnancy test for three days in a row in the morning. She got a blood test two days ago and it was positive and she is getting another tomorrow.  Here's the question.  Last night and tonight she took a test and they were negative. What does that mean?

Sunday morning 1st positive urine test
Monday morning positive urine test and got blood taken
Tuesday morning positive urine test positive blood test
Tuesday night negative urine test
Wednesday morning positive urine test
Wednesday night negative urine test
???

[AmandaD85]

It may be that the hormone that makes the urine test show positive is not strong enough in the evening. Thats why on the urine test it tells you to test with the first void of the day that is when the hormone is the strongest in the urine due to holding the urine all night. As we void so often during the day not allowing to show the hormone on the night test. The blood test is the most accurate as it test the actual hormone in the body and gives a exact number which can help determine how far along a woman is as well.

[RockRose]

I'm curious as to why she's taking all these tests - and two blood tests administered by the doctor 3 days apart.  Is there an obvious problem with the pregnancy?  

I agree with Amanda,  all the negative tests have been in the evening,  but urine pregnancy test instructions say use first morning urine.  And they have all been positive.  

[joebaby]

I guess we are just worried.  We have been trying fof over a year and finally the week before we were going to a fertility specialist we get this.  I know she is worried about the blood levels doubling.