Your baby

Doctors and midwives start the pregnancy clock on the first day of a woman's last menstrual period (LMP). Actually, this 40-week timetable isn't a conspiracy to confuse you, it’s how professionals take account of all the natural variations in women's cycles.

The progress of pregnancy is often discussed in terms of trimesters, with pregnancy divided into three parts, with each part lasting a little more than 3 months long.

Using the 40-week calendar, your first trimester will last from the first day of your last period and will go through the end of week 13. Your second trimester begins on week 14 of your 40-week calendar. Your third trimester will start with week 25 and end when your baby is born, somewhere around week 40.

Your progress

Getting the news you're pregnant can be thrilling or shocking, or both. Most women first discover they're pregnant when they read a positive symbol on a home pregnancy test.

Your next step will probably be to make an appointment with your GP or family planning clinic to confirm that you're pregnant, and to get that very important piece of news: when you can expect your baby to arrive. It's important to remember that your due date is simply an estimate. Only about five percent of babies actually arrive on the day they're due. In reality, your baby could arrive two weeks before or after your due date and still be considered on time.

Some babies come sooner than that. Sadly about 15% of pregnancies ends in a miscarriage, usually before the heartbeat is detected, and one out of eight babies arrives prematurely.

Although the waiting and uncertainty of not knowing exactly when your baby will arrive can be unnerving, the best plan is to stay flexible and to always be prepared for the unexpected.

What to think about

If you can remember when your last period started, you can calculate your own due date quite easily. Simply add seven days to the first day of your last menstrual period, and count forward nine months.

If your menstrual cycle is normally longer or shorter than 28 days you'll need to add or subtract any additional days, more or less than 28, to get a more accurate final date.