How long does a pregnancy last?

The length of a pregnancy may sound like a basic question, but one worth asking as there is often some confusion around it.

The length of a typical pregnancy is approximately:
- 38 weeks from time of conception
- 40 weeks from last menstrual period (or LMP as some call it)

Why are there two different lengths? 

Doctors use the LMP date as it is often the most solid point of reference. Many people don't know the actual date of conception, so the LMP allows for consistent comparisons from person to person amond medical professionals. The 38 week timeline is often referred to as the gestational age.

I thought a pregnancy was 9 months long? Isn't 9 months 36 weeks?

Well, it is 9 months, but the average month is 4.3 weeks long, not 4.
- (52 weeks / 12 months = 4.333 weeks per month.)  
- 4.333 weeks x 9 months = 38.99 weeks
So 9 months is just about the average of the 38 and 40 week numbers.
 To sum up, doctors talk about pregnancy on a 40 week timeline, the actual gestational time is about 38 weeks. All the being said, there are some variances in baby due date accuracy.

How accurate are home pregnancy tests?

Now that you have (or have not) seen a couple of blue lines on a home pregnancy test, you may be asking yourself just how accurate are pee-on-a-stick home pregnancy tests, anyway? 

For those of you who are flipping out right now and can't really focus, read the bulleted items below. For those of you who can concentrate, continue reading below for more of an explanation.

1. Most pregnancy tests claim to be 99%+ accurate.

2. Pregnancy tests should be taken in the morning, with the day's first urination. hCG,  the marker detected in home pregnancy test is most concentrated in the urine at that point, increasing the likelihood for an accurate result.

3. Take at least two home pregnancy tests several days apart. You're most likely to get an accurate result over the course of two or three tests spaced several days apart. This will help eliminate false negatives due to low hCG levels, and false positives due to faulty tests, improper use or other factors.

4. The longer you wait to take a test, the more likely it will be accurate. Aim for 1 week after a missed period. Again, hCG levels are more likely to be at sufficiently detectable levels in the case of pregnancy.

5. False negatives are more common than false positives. You're more likely to be told you're not pregnant when you really are, but false positives are also possible due to a number of factors.

So for some more explanation.

Home pregnancy test work by detecting hCG (Human Chorionic Gonadotropin) in a woman's urine.  hCH is a "glyco protein hormone made by the developing embryo soon after conception," so its presence is a good way of detecting pregnancy. The tests are essentially dumb, all they can do is measure hCG and turn a color.

The answer on most boxes is that they are 97% -99%+ accurate. I am skeptical of this as that 99% claim is based on a lab trials, not on aggregate real world use.  False negatives are more common than false positives, but both are possible due to a number of factors. 

Knowing that, it is essential to take the test at the proper time to optimize your results. (See # 2, 3, 4 above.) Some tests claim to determine pregnancy even before a woman has missed her first period. While this may be true, I would argue that one is more likely to have a false-negative (the test says you're not pregnant when you actually are) if taken very early as hCG levels are still very low at that point.   hCG levels increase rapidly in pregnant women, roughly doubling every couple of days. With that in mind, it makes sense that the longer you wait to take a home pregnancy test, the more likely you are to have an accurate result. 

I know that it is often incredibly difficult to wait that long to find out, so if you do take an early test you should follow up with at least one more in another few days.

Always follow the instructions on the box exactly, and good luck whatever the result.