First trimester
4 weeks pregnant
Baby is about the size of a poppy seed.
Week 4 of 40 · in your first trimester · Updated
Your baby this week
Implantation happens. The tiny ball of cells starts forming the embryo and placenta.
- The blastocyst splits into two parts: the inner cell mass becomes the embryo, and the outer layer becomes the placenta.
- The amniotic sac and yolk sac begin forming around the embryo.
- Three distinct layers — endoderm, mesoderm, ectoderm — start forming this week. Each layer will become specific organ systems.
Your body this week
A pregnancy test may finally turn positive! Some spotting (implantation bleeding) is normal.
- hCG levels are doubling every 48–72 hours. A home pregnancy test on the day of your missed period (or a few days after) should turn positive.
- Your body is releasing relaxin, which softens ligaments — you may already feel slightly more flexible.
- Symptoms can start subtly: fatigue, sore breasts, mild nausea, frequent urination.
Common symptoms
- Missed period
- Sore breasts
- Fatigue
- Mild nausea
- Frequent urination
- Light spotting
Things to do this week
- Take a home pregnancy test
- Book your first prenatal appointment (typically scheduled for 8–10 weeks)
- Confirm your last menstrual period date
Tip
Schedule your first prenatal appointment.
For partners this week
A positive test!
- Celebrate in whatever way feels right to your partner.
- Offer to help book the first prenatal appointment and add it to your calendar.
- Talk about who you'll tell, and when.
Typical visits around this week
Based on a typical schedule in Ontario. Your doctor or midwife sets the actual timing.
Coming up
First prenatal visit
Weeks 6–12
Book with your doctor or midwife as soon as you know you're pregnant. This is usually the longest visit: your health and family history, medications, blood pressure and weight, and routine blood and urine tests (including your blood type and Rh factor). Your provider will also talk about your due date and the screening options ahead.
Questions to ask →Questions to ask ↑
- When is my estimated due date, and how was it worked out?
- Are my current medications and supplements okay to keep taking?
- Who will care for me during pregnancy and birth, and where would I give birth?
- Who do I call with questions or worries between visits?
- Which vaccines should I plan for this pregnancy?
Dating ultrasound
Weeks 7–12
An early ultrasound that measures the baby to confirm how far along you are. Dating is most accurate between about 7 and 12 weeks, and the scan also confirms the pregnancy is in the uterus.
Questions to ask →Questions to ask ↑
- Does this scan change my due date?
- Can my partner come into the room?
- Do I need a full bladder, and how should I prepare?
- When will my provider get the results?
When to call your doctor or midwife
Bleeding, fluid leaking, a bad headache or vision changes, or baby moving less than usual all need a call. See the warning signs and who to call →
Follow your pregnancy week by week in MyBabyBase.
A due-date countdown, a mood and symptom journal, appointments, and a safe place for your documents — all in one hub.
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Further reading from health authorities
- Pregnancy — Government of Canada (Public Health Agency of Canada)
- Your pregnancy — Pregnancy Info, from the Society of Obstetricians and Gynaecologists of Canada (SOGC)
- Week-by-week guide to pregnancy — NHS (UK)
- Pregnancy — American College of Obstetricians and Gynecologists (ACOG)