Week 3 of Pregnancy: Symptoms, First Signs, and Changes

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A key moment occurs this week: fertilization. If a sperm successfully fertilizes the egg, the formation of the zygote begins, which will travel to the uterus to implant in the coming days. Although there are no obvious symptoms yet, pregnancy has already begun at the cellular level.

Symptoms

Implantation bleeding (light and pink or brown in color)

Occurs when the embryo attaches to the uterus.
Lasts 1-2 days and is lighter than menstruation.

Tiredness or fatigue

Your body begins to expend more energy in the implantation process.
There may be a feeling of exhaustion without apparent reason.

Increased basal temperature

If you keep a basal temperature record, you will notice that it remains elevated after ovulation.
This is an early sign of pregnancy due to progesterone.

Cramps or discomfort in the lower abdomen

Similar to menstrual cramps, but milder.
They are due to hormonal changes and implantation.

Changes in the digestive system (bloating and gas)

Progesterone relaxes the muscles of the digestive system, which can cause bloating or gas.
You may notice your abdomen more swollen than normal.

Greater sensitivity to smells

Some women notice that certain smells seem more intense or unpleasant.
It may cause mild nausea in some women.

Changes in appetite or cravings

Some women feel hungrier or begin to have preferences for certain foods.
Others may notice a slight aversion to foods they previously enjoyed.

Mood swings or greater emotional sensitivity

The increase in hormones can make you feel more sensitive, irritable, or emotional.
You may notice mood swings without apparent reason.

Embryo size

In week 3 of pregnancy, the embryo is actually a group of rapidly dividing cells, known as a blastocyst, which does not yet have a defined baby shape.

Size: Approximately 0.1 mm (similar to the head of a pin).
Developmental stage: The blastocyst has about 100 cells and is implanting in the uterus.

Despite its tiny size, it already contains all the genetic information that will define its physical and biological characteristics.

What to do this week?

- Maintain a healthy diet
- Start taking prenatal vitamins
- Keep your body hydrated
- Avoid harmful substances
- Perform moderate exercise
- Track your menstrual cycle
- Schedule an appointment with your gynecologist
- Avoid stress

Nutrition in week 3 of pregnancy

Folic acid (Vitamin B9)
Prevents neural tube defects.
Important for DNA formation and cell growth.
Sources: Spinach, broccoli, lentils, asparagus, avocado, oranges.

Iron
Helps with red blood cell production and prevents anemia.
Sources: Lean meats, lentils, spinach, chickpeas, quinoa, liver.
Tip: Combine it with vitamin C (oranges, strawberries) for better absorption.

Calcium
Essential for the development of the baby’s bones.
Sources: Dairy products, almonds, tofu, broccoli, sardines.
Recommended dose: 1,000 mg daily.

Proteins
Fundamental for cell development and embryo growth.
Sources: Chicken, low-mercury fish, eggs, legumes, Greek yogurt.
Healthy fats (Omega-3)
Promote the development of the baby’s brain and vision.
Sources: Salmon, chia, walnuts, flaxseed, olive oil.

Hydration
Maintains circulation and prevents fatigue.
Drink at least 2 liters of water per day.
Avoid sugary drinks and sodas.

Light weight strength exercises (2-3 times per week)
Use light dumbbells or resistance bands.
Strengthens arms, back, and legs to support the baby’s growth.

Exercise in week 3 of pregnancy

Staying active will improve circulation, reduce stress, and strengthen your body for the months ahead.

Remember: If you were already exercising before, you can continue with your routine, adapting it according to your energy. If you are a beginner, start with low-impact activities.

Walking (30-40 min, 4-5 days a week)
Activates circulation and oxygenates the body without strong impact.
Prenatal yoga or Pilates (2-3 times per week)
Improves flexibility and strengthens the pelvic floor.
Promotes relaxation and reduces muscle tension.

Swimming or water exercises (2 times per week)
Excellent for strengthening muscles without impact on the joints.

Deep breathing and meditation (5-10 min daily)
Relaxes the body and reduces stress.

Things you should NOT do in week 3 of pregnancy

Consume alcohol, tobacco, or drugs: They can affect fertility and implantation.
Self-medicate: Consult a doctor before taking any medication.
Expose yourself to toxic substances: Avoid pesticides or strong chemicals.
Do not do high-impact exercises
Do not neglect rest or stress management

Frequently asked questions about week 3 of pregnancy

Can I know if I am pregnant in week 3?

Not yet. The hCG hormone, which pregnancy tests detect, is still at very low levels. It is best to wait until week 4 to take a reliable test.

What size is the embryo in week 3?

It is not yet an embryo, but a zygote, a microscopic cell of approximately 0.1 mm, which will divide rapidly in the coming days.

Should I change my diet at this stage?

Yes, it is a good time to strengthen your nutrition with:
-Folic acid (essential for the baby's development).
-Proteins, calcium, and iron (for a healthy pregnancy).
-Plenty of water (to avoid fatigue and improve circulation).

Is it normal to have spotting in week 3?

It is not common at this stage. If there is slight bleeding, it could be implantation (more common in week 4). If it is heavy or with intense pain, consult your doctor.

Can I continue exercising?

Yes, but choose low-impact routines, such as walking, swimming, or prenatal yoga.

When does the embryo implant in the uterus?

This happens at the end of week 3 or the beginning of week 4.