A normal pregnancy lasts around 37 to 42 weeks, or an average of 40 weeks from the first day of your last period. Pregnancy is often divided into thirds, called trimesters. It's a useful way to help describe the changes that happen during pregnancy. The first trimester is week one to week 12, the second is week 13 to week 27, and the third is week 28 to week 40.
One of your ovaries releases an egg around 14 days before your period. This is called ovulation. It's usually in the middle of a 28-day menstrual cycle, but can be later if you have longer cycles and earlier if your cycle is shorter. The egg can be fertilised by sperm in one of your fallopian tubes. Conception is when the fertilised egg then travels along the tube and implants in your womb (uterus). Implantation happens about six days after fertilisation. The fertilised egg grows and is called an embryo.
When you become pregnant, your womb doesn't shed its lining as it normally does at the end of a menstrual cycle, so you don't have a period each month. A missed period is one of the early signs of pregnancy.
You may be able to buy a pregnancy test from a pharmacy (chemist) or supermarket. These can test for pregnancy from the first day of a missed period and, if the test is positive, then it indicates that you are pregnant. A doctor or clinic may also be able to arrange a pregnancy test for you. Some testing kits can give a result even before you miss a period.
The baby and surrounding structures in late pregnancy
Weight gain in pregnancy
You will put on weight during your pregnancy - the exact amount varies from woman to woman. You will be weighed early on in your pregnancy. Unless you are very overweight or very underweight, you probably won't be weighed again because it doesn't necessarily give any useful information about your developing baby.
If you're concerned about your weight gain, talk to a doctor or midwife. He or she will be able to tell you whether it is too much or too little.
The weight you gain during pregnancy isn't the same as 'getting fat'. The extra weight is made up of:
- the developing baby, placenta and amniotic fluid
- the growth of your womb and breasts
- the increased blood in your circulation
- water retention
- essential fat stores