Pregnancy is divided into three stages, or trimesters, each about 3 months in duration. Each trimester brings about significant changes in the mother’s body and progressive development and growth of the baby. During the first 8weeks of pregnancy, the developing baby is called an embryo. After 8 weeks, it is called a fetus.
Embryo at 4 weeks
At 1 month old, the embryo has bulges on either side of its head where the eyes are beginning to form.
First trimester (weeks 1–12)
There are few visible changes in the mother’s body during this trimester. However, her heart rate increases by about 8beats per minute to increase the blood circulation. Most of the growing fetus’s major organs, such as the heart and the brain, become fully developed during the first 3 months.
From embryo to fetus
The first trimester is crucial in the baby’s development; 8 weeks after fertilization, the embryo has developed most of its organs and is now called a fetus. During the third month, it more than doubles in length.
The mother at 12 weeks
At 12 weeks, the mother’s waistline is only slightly thickened. The breasts are tender and the areola, the area around the nipple, becomes darker in colour.
Second trimester (weeks 13–28)
The mother may experience backache as she copes with the growing weight of the fetus. Her appetite tends to increase. Fetal movements are first perceived at 18–20 weeks as fluttering sensations in the mother’s abdomen.
The mother at 24 weeks
The mother’s abdomen swells as the fetus grows. There may be discharge from the nipples, caused by enlarging lobules (milk-producing glands) in the breasts.
The fetus grows in size and accumulates a layer of fat, and its internal organs become more complex. Its sense of hearing is now developed.
Fetus at 20 weeks
At this stage, the hands are well developed and the fetus is able to flex its fingers. The fetus now has recognizable features and the profile of the face shows the forehead, nose, lips, and chin.
Third trimester (weeks 29–40)
During the third trimester, the mother rapidly gains weight as the fetus undergoes a growth spurt. The top of the uterus is high in the abdomen, almost to the level of the breastbone. This can cause slight shortness of breath due to compression of the mother’s lungs.
Ultrasound scan of the fetus
The face of a fully grown fetus is clearly visible on this ultrasound scan. The head is lying sideways and the eyes and lips are closed.
The fetus at 36 weeks
The fetus is now fully formed, weighs about 3 kg (7 lb), and is about 50 cm (20 in) in length.
The mother at 36 weeks
The growing uterus presses on the intestines. The stomach may also be compressed, causing heartburn.