4 weeks pregnant

What's happening with your baby

At 4 weeks, your baby is a blastocyst — a hollow ball of about 200 cells that has just finished implanting in the lining of your uterus. The cells are starting to divide into two layers. The inner layer will become the embryo; the outer layer will become the placenta.

There's no recognizable shape yet — no head, no limbs, no organs. What is happening is the foundation: the cells that will eventually form every part of your baby are settling into their roles.

Your body is producing hCG (human chorionic gonadotropin) for the first time. It's hCG that home pregnancy tests detect, and it's hCG that will sustain the corpus luteum (the structure in your ovary that maintains pregnancy until the placenta takes over).

Common symptoms at 4 weeks

Most people feel either nothing or very mild signs at 4 weeks. The earliest symptoms tend to mimic PMS, which is why it's so hard to tell early on.

If you do notice anything, it's likely to be: mild cramping or pulling sensations as your uterus begins to change, breast tenderness, slight fatigue, mood changes, and possibly very light spotting (implantation bleeding) — though most pregnancies don't include any spotting.

If you've taken a home pregnancy test and gotten a positive at 4 weeks, that's actually quite early — hCG levels are just at the detection threshold. A faint line is real. If your test was negative but you suspect pregnancy, retest in 2–3 days; hCG roughly doubles every 48 hours.

What to do this week

Start taking a prenatal vitamin with at least 400 micrograms of folic acid if you haven't already. Folic acid is most important in the first few weeks — it dramatically reduces the risk of neural tube defects.

Schedule your first prenatal visit. Most providers see new patients between weeks 8 and 12, so you can call now to book the appointment. If you have a chronic condition (diabetes, thyroid disease, hypertension) or a history of pregnancy loss, mention that — your provider may want to see you sooner.

Avoid alcohol, smoking, and recreational drugs. If you're on prescription medications, talk to your provider before stopping anything — but flag the pregnancy so they can review whether any of them need to change.

When to call your provider

A positive test is reason enough to call and book your first appointment. Most providers don't need to see you in person immediately at 4 weeks.

Call sooner if you experience severe abdominal pain, heavy bleeding (more than a light spot), or pain on one side that could indicate ectopic pregnancy. These are uncommon but worth ruling out early.

Frequently asked questions

Sources and medical references

• ACOG — Your Pregnancy and Childbirth: Month to Month
• Mayo Clinic — Pregnancy week-by-week
• NHS — Pregnancy: week-by-week guide