5 weeks pregnant: baby is the size of a sesame seed

Your baby this week

This is a week of rapid construction. The embryo is a small curved disc. A groove runs along its back, and its edges are folding together to form the neural tube. This tube will become your baby's brain and spinal cord.

The circulatory system is starting to form too. A simple heart tube develops and, around the end of this week or the start of the next, it begins to beat. The first buds that will become arms and legs are not visible yet, but the cells that will form them are already set aside.

How it happens. Layers of cells that formed during implantation signal to each other, and each layer turns on a different set of genes. One layer becomes the nervous system and skin, another becomes the heart, muscle and bone, and the third becomes the lining of the gut and lungs. The neural tube needs folate (the natural form of folic acid) to close properly. This is why supplements are advised before and during early pregnancy.

Your body this week

Your hormone levels are climbing quickly. Understanding what each hormone does explains most of what you feel.

hCG comes from the developing placenta and keeps progesterone production going. Its level rises fast in the first weeks, and it is thought to be linked with nausea, although the exact cause is not fully understood.

Progesterone relaxes smooth muscle throughout your body. That helps the uterus stay calm, but it also slows digestion, which can cause bloating, constipation and heartburn. Progesterone also tends to make you sleepy.

Estrogen encourages growth in the breasts and uterus and increases blood flow, which is why your breasts may feel full and tender.

Your body is also doing a lot of quiet work. It is increasing blood volume, building the placenta and supporting an embryo that is growing every day. Fatigue is your body's way of telling you that it needs more rest than usual.

Managing first-trimester fatigue

Fatigue is one of the most common early symptoms, and it can be hard to manage because few people know you are pregnant yet. These steps can help:

• Go to bed earlier. An extra hour or two a night makes a noticeable difference for many people.
• Take a short nap. Twenty to thirty minutes in the afternoon is often enough. Longer naps can affect sleep at night.
• Eat regularly. Small meals or snacks every two to three hours keep blood sugar steady. Include protein, such as eggs, yogurt, beans or nuts.
• Include iron-rich foods. Lean meat, lentils, beans and leafy greens support your blood supply. Vitamin C in the same meal, for example from fruit or peppers, helps you absorb iron from plant foods.
• Drink water through the day. Even mild dehydration adds to tiredness.
• Move gently. A short walk often lifts energy better than another hour on the couch.
• Lower your load. If you can, cut back on non-essential tasks for a few weeks.

If you are so tired that you cannot function, or if you feel faint, breathless or pale, mention it to your provider. A blood test can check for iron deficiency (anemia) and thyroid problems, both of which can cause fatigue.

Why you may feel nothing at all

Some people have no symptoms in the first weeks. That can feel worrying, and many people search for reassurance that the pregnancy is still going well. Symptoms vary enormously and do not track pregnancy health. If you are worried, tell your provider, who can arrange a check.

Can you hear a heartbeat at 5 weeks?

Most of the time, no. The heart tube is only beginning to beat, and a standard ultrasound may not pick it up yet. Many clinics wait until around 6 to 8 weeks for a first scan, since the heartbeat is easier to see then. If you have an early scan and no heartbeat is seen, it may be too early rather than a sign of a problem, and your provider may suggest repeating it in a week or two.