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Fertility Friend

Fertility Friend makes charting and graphing your BBT and other fertility signs easy and clear. All you have to do is enter your data on the included data entry form! It will also analyze your data, detect ovulation, and provide an accurate assessment of your fertility status on a given day!
You can read more about it and order the software from



Using Glass thermometer

Use a special basal temperature thermometer, available at drug stores, because it is more accurate than a fever thermometer. These thermometers only measure up to 100 degrees, so if you might be sick, use a fever thermometer instead. Shake it down the night before, over the bed so it won't break if you drop it. Take your temperature for five minutes. If you're not ready to get up, you can set the thermometer aside and read it later. The mercury in a good thermometer will not go down until you shake it down.



Using Digital thermometer

Dawn says:
I prefer my digital, since it gives me precise readouts and most digitals have memory so you can take your temp. , take your shower and come back to it to read it out. If I were to try to read a glass thermometer, I would have given myself mercury poisoning by now due to repeated droppings. Plus it has that lovely "beeping" which tells you when it's done and , as an added bonus, keeps you awake!!!



How to take the BBT?

You can either take your BBT orally or vaginally.

Leta3 says:
I took mine vaginally the first cycle, this cycle I switched to orally, my temps are eratic no matter which way I take them. I switched to orally taking them when my waking temps were so high. They ranged from 98.4 - 98.6. That made my cover-line 98.7. I thought with the high waking temps that I had a thyroid problem. Doctor even did some blood work. It was just that I didn't know what a big difference it was between the 2. Taking my temps orally they range from 97.4 to 97.9 post Ovulation. Just wanted to let you know so that if you do, dont be surprised if they are high

EileenB says:
I've done both. I switched to vaginally about 3 cycles ago. I prefer it, first of all because I have a tendency to sleep with my mouth open, which ruins the accuracy of an oral reading right after you wake up (how do you know if your mouth was open or closed if you were asleep?). Secondly, I saw that my temps tended to CREEP up orally, but vaginally I get a consistently quick reading, usually in about 40 seconds. Plus, you can fall asleep without having the thermometer drop out of your mouth! Much more comfortable too.



When to take the BBT?

Dawn says:
First thing in the morning. Do not move, do not get up to pee, do nothing but pop the thermometer in your mouth or vaginally, if that's your preference.



What if I wake up early?

Add 1/10 of a degree for every half hour that you wake up earlier than your usual time.



What if I wake up late?

Subtract 1/10 of a degree for every half hour that you sleep over your usual time.



What can affect the accuracy of BBT?



What if I get up in the middle of the night?

Wampumpeag says:
According to TCOYF, you need three hours of uninterrrupted sleep to get an accurated BBT. So if you get up to pee at 2am, but don't take your temp until 6AM, you should be fine.
That said, however, my own experience has led me to believe that consistency is just as important - I always took my temperature after one morning activity (nursing), a no-no according to *the rules*, and I still had a textbook cycle (low 97's pre-O, drop to 96.9 at O, and up to mid 98's post-O), except for the one day the baby slept in the guestroom with the DH (drop of .5). So I guess I would say, if you have to get up to pee, make sure you get up ever night, or note those nights that you don't.



Coverline, what is it?

The coverline is what separate your pre-ovulation days to your post-ovulation days.

Laura (aka LkGls) says:
When charting your temps, after your temp spike,you draw a "coverline" over the highest of the last 6 day temps. Your spike and all suceeding temps (until day or 2 before AF) should be above this line. (Called bi-phasic temps . . . all before O should be below the coverline and all after should be above).

Heather (aka mabriam) explains it more with an example:
Let's say you have these temps:
Day 7 8 9 10 11 12 13 14 Ovulation 15 16 17
Temp 97.3 97.2 97.4 97.2 97.4 97.3 97.1 97.6 97.8 98.0 97.9

Okay, you can clearly see the .5 degree jump on day 14, indicating ovulation. To get your coverline, you look at the 6 temps previous to O and add .1 degree to the highest one. In this case, the highest one is 97.4, so the coverline is 97.5. All clear?



BBT BEFORE Ovulation

When AF begins, or a day or two earlier, your BBT will fall to its normal pre-ovulation level. It will remain at the lower level until it begins to rise after ovulation.



BBT DURING Ovulation

The temperature drops at its lowest level signaling that Ovulation will occure within the next 24 to 48 hours.



BBT AFTER Ovulation

After ovulation, your temperature rises. You can be sure that ovulation has occurred when you observe three normal temperatures that are higher than the previous six temperatures (coverline), with the third one being at least 4/10 of a degree higher than the coverline.



BBT IF Pregnant

If you are pregnant, your temperature will stay at the higher level. If it stays at that level for seven days more than your normal luteal phase, it is a positive sign of pregnancy.

dorothym says:
The Art of Natural Family Planning says that if your temp stays up for 21 days (NOT 18) you have a 99% certainty of pregnancy. Another way to figure it out is to add 7 days to your normal luteal phase. Mine is 16 days, so I would need to have 23 days of high temps to be sure.




Bleeding could occur while the BBT is high for any of several reasons. It could be staying up during AF because of an infection or illness. If you are pregnant, you could have implantation bleeding. In that case, it will not fall after the bleeding stops. If your BBT is up for the first few days of AF and then falls, it is called "irregular shedding". This is common in the premenopausal years. In younger women, it is a sign of luteal phase inadequacy.



BBT is high BEFORE Ovulation

MFS says:
Restlessness, illness, more covers than usual, hot weather, bad sleep - all these have made my temps jump for a day. TCOYF calls these "rule of thumb" temps, and you can just pretend they're not there when looking at the overall pattern (putting your thumb over it), and pretty much ignore it, as long as the rest of your temps surrounding it are more in the normal range. I wouldn't worry unless they stay up for days.



BBT drops AFTER Ovulation

Most women experience a drop of temperature just few days after Ovulation. This is mainly due to the fact that the progesterone level are not getting stronger yet against the dominance of the Eustrogen. Read the section about Anovulatory under the topic Ovulation for a detailed explanation of the possibilities that could occur.



BBT doesn' match CM-watching

Carmeli says:
TCOYF says that the temp delay is due to the body's delayed reaction to progesterone. That would explain ready's apparently short luteal phase. She may have actually O'd earlier than the temp delay reflected.
Another book i found entitled "your fertility signals" (i don't remember the author, some Dr.) says that sometimes ovulation occurs LATER than CM and cervical position predicts. That would explain why i had EW CM five days before my temp shift but still had a normal luteal phase of 12 high temps.
The last book i read was a handbook on the fertility awareness method written and published in great britain (gosh i should keep better track of my references). it said that all three indicators, namely BBT, CM and cervical position, just give an APPROXIMATE time of ovulation but the ACTUAL time of ovulation cannot be absolutely determined by those indicators alone. for example, ovulation could occur days before, just before or one day after the temp shift. CM and cervical position could be ovulatory but the body could just be preparing to ovulate.
In short, although our bodies give all these signals saying that it is your fertile time, there is no definite interpretation of those signals. so the only solution is to BD, BD and BD while all or even only one of those signals say GO and then some! there is no 100% solution, we can only maximize our chances.



Triphasic BBT?

Triphasic temperatures occur when you have a second level of high temperatures during the Luteal Phase. Most women associate the Triphasic BBT with a sign of pregnancy, but this is not always the case (I am Triphasic every month) because the Triphasic temp is mainly associated with high levels of Progesterone hormones. With high progesterone hormones you can either be pregnant or not, so don't take Triphasic temps as a definitive sign of Pregnancy, unless you have more than 18 days of high temperatures during the post-cycle with a positive HPT (even if faint line).

MFS also states:
I've had triphasic temps and not been pregnant, and when I did get pregnant, I did not have triphasic temps.

Scoon did a survey on people who had triphasic temps but weren't PG, here is her summary:
I am afraid the survey is less definitive than I had hoped. Several people who had triphasic temps did report other signs that indicated they had probably had a fertilized egg that had either failed to implant or failed shortly after implantation. If you have triphazic temps and also either a faintly positive test at some point or a late AF or an unusually bad AF I think there is a very good chance that what happened is that there was a fertilized egg that for some reason didn't make it. The good news is that people who go through this, disappointing as it is, seem to have a higher than average success rate the next month.

Then there are at least a few people who really do go triphasic every month and show no other signs of being PG.

Then there are a few people who maybe didn't really go triphasic. Temps often have a certain amount of scatter, especially if taken orally, and this can confuse the issue. There needs to be a definition of triphasic. To get the best definition one would have to do a lot of research. As a starting point, I suggest having to have at least 4 temps at the initial post-ovulation level, followed by at least 6 temps at a level that is at least 0.3 degrees F higher than the previous level.

Finally, mine own experience. I had clearly triphasic temps that fit the above definition. AF was 3 days late. I had one faint positive HPT. I also had a blood test done at 13 dpo. It was barely positive, but so low they didn't think it would last. (By the time I got the results I could have told them that.) So, it looks like in my case I was PG for a little while, probably it never implanted. However, the bad news is that I insisted on a progesterone test at the same time, which came out VERY low. This may be the reason implantation failed. So, what does this mean for everybody else? If you have been trying for more than 6 months and have had one or more months of triphasic temps and have not gotten PG, you might want to consider getting progesterone testing done. It is pretty easy to do and could make you quest shorter.



Where to get the chart from the internet?

A complete one from
or a very basic one from



When to start counting DPO?

Count the first high temp (*after* ovulation) as 1 dpo.



Help on Fahrenheit/Centigrade conversion

Teylyn explains it as follow:
To convert F to C do (F = the temp in Fahrenheit)
5 * (F - 32) / 9
Example: 5 * (98.8 - 32) / 9 = 37.11

To convert C to F do (C = the temp in Celsius)
9 * C / 5 + 32
Example: 9 * 36.1 / 5 + 32= 96.98

Basic rule of thumb seems to be: 0.1 degrees Celsius equals 0.18 degrees Fahrenheit, 0.1 Fahrenheit is approx. 0.055 Celsius.

For everyone who does not like to do the maths themselves, here comes a little table:

 C --->  F               F  ---> C
  35,0    95,00           95,0    35,00
  35,5    95,90           95,5    35,28
  36,0    96,80           96,0    35,56
  36,1    96,98           96,1    35,61
  36,2    97,16           96,2    35,67
  36,3    97,34           96,3    35,72
  36,4    97,52           96,4    35,78
  36,5    97,70           96,5    35,83
  36,6    97,88           96,6    35,89
  36,7    98,06           96,7    35,94
  36,8    98,24           96,8    36,00
  36,9    98,42           96,9    36,06
  37,0    98,60           97,0    36,11
  37,1    98,78           97,1    36,17
  37,2    98,96           97,2    36,22
  37,3    99,14           97,3    36,28
  37,4    99,32           97,4    36,33
  37,5    99,50           97,5    36,39
  37,6    99,68           97,6    36,44
  37,7    99,86           97,7    36,50
  37,8    100,04          97,8    36,56
  37,9    100,22          97,9    36,61
  38,0    100,40          98,0    36,67
  38,1    100,58          98,1    36,72
  38,2    100,76          98,2    36,78
  38,3    100,94          98,3    36,83
  38,4    101,12          98,4    36,89
  38,5    101,30          98,5    36,94
                          98,6    37,00
                          98,7    37,06
                          98,8    37,11
                          98,9    37,17
                          99,0    37,22
                          99,1    37,28
                          99,2    37,33
                          99,3    37,39
                          99,4    37,44
                          99,5    37,50
                          99,6    37,56
                          99,7    37,61
                          99,8    37,67
                          99,9    37,72
                          100,0   37,78
                          100,1   37,83
                          100,2   37,89
                          100,3   37,94
                          100,4   38,00
                          100,5   38,06
                          100,6   38,11
                          100,7   38,17
                          100,8   38,22
                          100,9   38,28
                          101,0   38,33
                          101,1   38,39
                          101,2   38,44
                          101,3   38,50



BBT or CM, what's the best?

See answer under Cervical Mucus (CM) topic



over 18 DPO and not pregnant?

Although most think that 18 days of hight temperature is a sure sign of pregnancy, this is actually not the exact rule (read the item under BBT IF Pregnant). Here is my experience.
One of my cycle I had 20 hight temps and still was not pg (not normal for me after 2 years of charting). I was patient enough to wait and test until I was 19 DPO just to found at my big surprise that it was negative. I had all pregnancy symptoms and cramping started at about 7 DPO which made me believe that sure something is happening, but still no pg. My temps went triphasic at a very early stage (at 3 DPO) very unusual for me, but was still no pg. So I opened TCOYF book last night (I didn't read it since ages) and started to look for some answers. She says that 18 high temps are sure a pg indication and insisted on this. But I kept on reading the appendix at the "Troubleshouting" section as I was sure that there is more to the "18 hight temps" rule then meets the eye. And sure enough in one of the paragraphe she stated that "18 high temps" can also indicate a Corpus Luteum Cyst. The book didn't go into details about it but as I understand it the Corpus Luteum doesn't disintegrate when it should be (when AF is due) because it doesn't get the right message from the brain (pituitory gland in the brain) because of hormonal imbalance, but stays there and continues on producing progesterone for more than 16 days (maximum luteal phase length) which make the temps stay high, and give all the pregnancy symptoms. The Corpus Luteum is considered in this case as a Cyst until AF comes where it disintegrates with the blood flow.
Next time I will wait until I am 7 days late to test.

Cat also expresses this important point:
Although TCOYF cites 18 DPO for pregnancy, all my NFP literature uses a 21 DPO rule. I would ask you to include it in the faq to help calm fears that there is something wrong when 18 dpo comes and goes with a negative test.