by Melanie Denyer

Created on: November 04, 2007

It seems one of the most common questions on fertility forums is precisely this - how can I tell if I'm pregnant? Or, the alternate way of posting it - do you think I might be pregnant, followed by a list of symptoms. And whether pregnancy is desired or feared by those asking the question, the basics are the same for all women.

First of all, how well do you know your own cycle? Not everyone is a textbook 28 days: some have shorter cycles, some have longer ones. Those with conditions such as PCOS or endometriosis may find it all but impossible to calculate their cycle in any meaningful way. But whether you're trying to get pregnant, or trying to avoid it, it's probably useful to have a quick revision of your high school sexual biology class.

On day one of your cycle, your period starts, and your body prepares itself for another reproductive cycle by starting to release follicle stimulating hormone (FSH) and luteinising hormone (LH) in order to encourage the ovaries to ripen an egg. In your ovaries, an egg starts to grow, waiting for the hormonal surge that will cause its release. The precise time in the month that this occurs is what varies from woman to woman, but in all cases is signalled by a surge in the LH your body has been producing since day one of the cycle. This is the hormone ovulation predictor kits are testing for.

Within 24-48 hours of the LH surge, you ovulate and the egg is released from the ovaries to start its journey down the fallopian tubes towards the uterus. At this point, there is a 24 hour window in which the egg may be fertilised, should it happen across a friendly sperm along the way. At the same time as the egg is released, the body starts producing progesterone to prepare the womb lining for a new pregnancy, and you enter the luteal phase of your cycle. This is the one part of the cycle that is the same length for most women. In most cases, it will be between 12 and 16 days in length, and this is the time period in which your egg has a chance to be fertilised, make its way to the womb and implant. If there is no pregnancy, the egg (or corpus luteum as it's known by this stage) will start to break down, your body will stop producing progesterone, and your next period will be triggered.

So the first way of telling whether you are likely to be pregnant is to think back to whether or not you had intercourse around the time you're likely to have ovulated: simply work out when your period was due, then count back 14 days as a rough guide,

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