Pregnancy, also known as gravidity or gestation, is the time during which one or more offspring develops inside a woman. A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy can occur by sexual intercourse or assisted reproductive technology.

It usually lasts around 40 weeks from the last menstrual period (LMP) and ends in childbirth. This is just over nine lunar months, where each month is about 29½ days. When measured from conception it is about 38 weeks. An embryo is the developing offspring during the first eight weeks following conception, after which, the term fetus is used until birth. Symptoms of early pregnancy may include missed periods, tender breasts, nausea and vomiting, hunger, and frequent urination. Pregnancy may be confirmed with a pregnancy test.

Pregnancy is typically divided into three trimesters. The first trimester is from week one through 12 and includes conception. Conception is when the sperm fertilizes the egg. The fertilized egg then travels down the fallopian tube and attaches to the inside of the uterus, where it begins to form the fetus and placenta.

The first trimester carries the highest risk of miscarriage, natural death of embryo or fetus. The second trimester is from week 13 through 28. Around the middle of the second trimester, movement of the fetus may be felt. At 28 weeks, more than 90% of babies can survive outside of the uterus if provided high-quality medical care. The third trimester is from 29 weeks through 40 weeks.

Figure Out When You're Ovulating

Use the chart below to determine when you're most likely to be ovulating, based on the length of your menstrual cycle. Find the length of your cycle in the left-hand column, and determine how many days after the beginning of your period you are ovulating, and when you're at your most fertile.

The best time to try and conceive is during the "fertile window" of the menstrual cycle, which is different for different women.

The fertile window

We're talking about the days in a woman's menstrual cycle when pregnancy is possible. Pregnancy is technically only possible during the five days before ovulation through to the day of ovulation.

These six days are the "fertile window" in a woman's cycle, and reflect the lifespan of sperm (5 days) and the lifespan of the ovum (24 hours).

If a woman has sex six or more days before she ovulates, the chance she will get pregnant is virtually zero. If she has sex five days before she ovulates, her probability of pregnancy is about 10%.

The probability of pregnancy rises steadily until the two days before and including the day of ovulation.

At the end of the "fertile window", the probability of pregnancy declines rapidly and by 12-24 hours after she ovulates, a woman is no longer able to get pregnant during that cycle.

For those women who are not aware of their "fertile window" or when they ovulate, sexual intercourse is recommended every 2 to 3 days to help optimise their chance of conceiving.

The likelihood of actually becoming pregnant is dramatically increased if you have intercourse in the three days leading up to and including ovulation. If a woman has sex on any of these three days, she has a 27-33% chance of becoming pregnant.

Myth of conception

The myth: The best time to try to conceive is day 14 of your menstrual cycle.

The truth: The only time a woman can conceive is during the "fertile window" of the menstrual cycle. This window will vary depending on the individual's cycle, however timing intercourse in the 6 days leading up to and including ovulation is the window with highest chance of conception.

What is ovulation?

Ovulation is when a mature egg is released from the ovary, moves down the fallopian tube, and is available in the fallopian tube to be fertilised.

Tracking your ovulation

Work out the length of your average menstrual cycle. Day one is the first day of the menstrual period and the last day is the day before the next period begins.

Ovulation happens about two weeks before the next expected period. So if your average menstrual cycle is 28 days, you ovulate around day 14.

Remember the "fertile window" is the six days leading up to and including ovulation.

The three days leading up to and including ovulation are the most fertile. Depending on your cycle length the most fertile days in the cycle varies:

• If you have 28 days between periods ovulation typically happens on day 14, and the most fertile days are days 12, 13, and 14.
• If you have longer cycles, say 35 days between periods, ovulation happens on day 21 and the most fertile days are days 19, 20, and 21.
• If you have shorter cycles, say 21 days between periods, ovulation happens on day 7 and the most fertile days are days 5, 6, and 7.

How do you know you're ovulating?

Women's cycles can vary and are not always as regular as clockwork, so to know that you are ovulating and on which day of your cycle you are ovulating, observe your fertility signs throughout your cycle and record them on a chart.

The most accurate methods of working out when ovulation is about to occur are:

• Keep an eye out for changes in your mucus. Around the time of ovulation, you may notice your vagina's mucus is clear, slick and slippery, the consistency of egg white. This is the best sign of when ovulation is actually happening. It's prime time for action.
• Use an ovulation predictor kit. You can start testing with your ovulation predictor kit a few days before your estimated day of ovulation. Subtract 17 days from your average cycle length and start testing from this day of your cycle, e.g. if you have a 28 day cycle, you would start testing from day 11. A positive result means you are going to ovulate within the next 24 to 36 hours.
• Record your basal body temperature (BBT) each day before getting out of bed. A special basal body temperature thermometer will ensure accurate measurement. Your BBT rises about half a degree Celsius after ovulation has occurred. By charting your temperature, it's easy to see when the rise in temperature and ovulation happens. This can help you work out your own pattern of ovulation. However, because at that stage ovulation has already passed, it does not help you pinpoint the fertile window but may guide you for next month.
• Use the ovulation calculator on this site. If you know the date of your last period, the length of your cycle and your cycle is regular, this will identify your ‘fertile window' and predicted ovulation date.

How to find out your most fertile days?

All you need to do is put in the first day of your last period. Then we need to know the average length of your menstrual cycle so we can calculate the window of time when you're most fertile. While the average cycle lasts 28 days, anything from 22 to 36 days is perfectly normal.

How soon can you take a pregnancy test?

Home pregnancy tests detect the presence of hCG in your urine. Some pregnancy tests are more sensitive than others. The more sensitive tests may be able to detect low levels of hCG as early as four days before your period is due, or seven days after you've conceived.