First trimester - 1-12 weeks

First trimester (week 1–week 12)

The first trimester lasts until you are 13 weeks pregnant, covering the first three months of pregnancy. During this time, the fertilized egg grows from a small group of cells into a foetus with some human features. While this trimester can be exciting, it's also when many people experience symptoms like morning sickness and fatigue.

Your body changes a lot during the first trimester. Hormonal changes affect almost all body systems, causing symptoms even in the early weeks. A missed period is a clear sign that you are pregnant.

As your body changes, you may need to adjust your daily routine. This could mean going to bed earlier or eating smaller, more frequent meals. Most of these discomforts usually disappear as your pregnancy continues. However, some women may not feel discomfort at all. If you have been pregnant before, your experience may be different this time. Every woman and every pregnancy are unique.

Below are some common symptoms that you might experience during this first stage of pregnancy.

• Missing your period.
• Having tender breasts.
• Needing to pee (urinate) more often than usual.
• Fatigue and feeling tired.
• Feeling sick or being sick. This is often called morning sickness, but it can happen at any time of day.
• Being constipated. Drinking lots of fluids and getting plenty of fibre can help.
• Extreme tiredness
• Tender, swollen breasts. Your nipples might also stick out.
• Cravings or distaste for certain foods
• Mood swings
• Headache
• Heartburn
• Weight gain or loss

A lot of these symptoms are likely to diminish as your pregnancy progresses. If you're having difficulty with any of your pregnancy symptoms, consult your GP or midwife. They might suggest that you take a prenatal vitamin. They will also provide you with a list of items to stay away from during your pregnancy. Some of these items consist of:

• Beverages containing alcohol.
• Tobacco products.
• Recreational drugs, including marijuana. Medical marijuana should be discussed with your pregnancy care provider.
• Certain foods like unpasteurized dairy, raw fish or fish high in mercury.

Your baby starts its journey as a small cluster of cells. Some of these cells develop into an embryo, while others become the placenta. The placenta connects to your uterus and is linked to your baby through the umbilical cord. Through the placenta and umbilical cord, oxygen, nutrients, and hormones from your bloodstream are delivered to your baby. The waste products from your baby flow back into your bloodstream for you to eliminate. In the initial months, all of your baby’s organs and body systems begin to develop. The cells responsible for forming your baby’s heart start to pump rhythmically. An ultrasound scan around the sixth week of pregnancy can detect your baby’s heartbeat. By week 12, your baby begins to resemble a small human being.

Month 1 (weeks 1 through 4)

• Week 1-2: The first two weeks of pregnancy are a time for preparation. During this period, your body starts to release more hormones, and your uterus gets ready for a possible pregnancy. At the end of the second week, your ovary releases an egg, which is called ovulation. If sperm meets the egg right after ovulation, pregnancy can begin.
• Week 3: Actual Fertilization occurs during the third week. A sperm and egg join and create a zygote.
• Week 4: As week 4 approaches, the small cluster of cells develops into a blastocyst and attaches itself to the lining of your uterus. The initial formation of what will develop into the placenta begins. A protective sac envelops the blastocyst, known as the amniotic sac, which offers cushioning to the foetus throughout pregnancy.

By the end of the fourth week, the blastocyst is about 2 millimetres (mm) long — the size of a poppy seed.

Month 2 (weeks 5 through 8)

The second month of pregnancy is typically when many individuals become aware that they are expecting. Pregnancy hormones increase significantly, and by around the fifth week, a home pregnancy test is likely to indicate a positive result. This period is when many start to experience pregnancy symptoms.

• Week 5: The neural tube, which includes the brain and spinal cord, forms. By the end of this week, the tiny heart will beat 110 times a minute. Arm and leg buds appear. Your baby is now an embryo and is one-twenty-fifth of an inch long. 
• Week 6: The small buds that will become arms and legs continue to grow. Blood cells start to form, and circulation begins. Ears, eyes, and the mouth begin to take shape. A vaginal ultrasound can likely find the heartbeat in the developing cluster of heart cells. 
• Week 7: Bones start to replace soft cartilage, and genitals begin to develop. The embryo's head is large compared to its body. Some people think the embryo looks like a small tadpole or seahorse because of its prominent tail (which will go away) and large head. 
• Week 8: All major organs and body systems are developing. The embryo has web-like hands and feet. The eyes become visible, and the ears begin to form. The umbilical cord is fully developed and helps carry oxygen and blood to the embryo.

By the end of eight weeks, your baby has developed into a foetus and resembles a more human form. The sex organs begin to develop, the eyes have shifted forward on the face, and eyelids have formed. At this stage, your baby is nearly 1 inch long, which is about the size of a black bean, and weighs less than one-eighth of an ounce.

Month 3 (weeks 9 through 12)

The third month of gestation marks the transition from embryo to foetus. This time is characterised by swift growth and advancement. The foetus begins to form unique facial characteristics, limbs, organs, bones, and muscles.

• Week 9: The initial development of teeth and taste buds is underway. Its muscles are developing, and its body shape is starting to look more human-like. However, its head still constitutes 50% of its overall length. Your healthcare provider might be able to detect its heartbeat using a Doppler ultrasound.
• Week 10: The arms, hands, fingers, feet, and toes are fully formed, and there are no more webbed fingers. Fingernails and toenails are beginning to develop, and the external ears are starting to take shape. The external genitals are also beginning to form, but it is too early to detect them on an ultrasound.
• Week 11: The foetus is beginning to explore its movements, such as opening and closing its fists and mouth. Its knees, elbows, and ankles are functioning, but it's still too early to feel any kicks. While the bones are hardening, the skin remains translucent. Additionally, the facial features are becoming more prominent.
• Week 12: All the organs, limbs, bones, and muscles are present and will continue to develop until they are fully functional. The circulatory, digestive, and urinary systems are actively working, and the liver is producing bile. The foetus is also drinking and excreting amniotic fluid.

At the end of the third month, the foetus measures about 2.5 to 3 inches long—approximately the size of a plum. The external sex organs have developed, indicating whether your baby is a boy or girl; however, this may still be unclear, and it won’t be visible on ultrasound for several more weeks. It is recommended to wait until the second trimester or later to determine the baby's sex, as waiting longer increases the likelihood of receiving an accurate result.

Most of the critical development has occurred by this point, which means the chance of miscarriage decreases significantly after 12 weeks, marking the end of the first trimester. Additionally, many people start to feel relief from morning sickness around this time.