Your embryo implants in your uterus

What to expect

This is when pregnancy truly begins. Fertilization happened about two weeks ago (gestational weeks count from your last period), but now the tiny embryo implants into your uterine lining. Some people feel a slight pinch or cramping during implantation, but most don't notice.

The placenta starts forming

While your baby is poppy-seed small, the placental cells are tunneling into your uterine wall like roots. These will become the lifeline that nourishes your baby for the next eight months. By week's end, primitive blood vessels begin forming between you two.

Hormones are about to surge. Progesterone and estrogen are rising to maintain the pregnancy, which might cause tender breasts and fatigue soon. Human chorionic gonadotropin (hCG), the pregnancy hormone detected by tests, doubles every 48 hours now.

• Your baby has three layers. The ectoderm (outer layer) will become skin and nervous system, mesoderm (middle) forms heart and muscles, endoderm (inner) develops lungs and digestive tract.
• Primitive circulation begins. Tiny blood vessels start linking embryo to maternal blood supply, your first physical connection.
• The amniotic sac forms. This fluid-filled bubble will cushion your baby through every kick and turn to come.

Things to be aware of

Most people don't feel pregnant yet. While implantation may have just occurred, it's common to have no physical symptoms at this stage. Don't worry if you're not 'feeling' pregnant, your body is working hard behind the scenes.

Early pregnancy tests can be tricky. If you test now, you might get a false negative. The pregnancy hormone hCG doubles every 48 hours, so waiting until after your missed period (around week 5) gives more reliable results.

Spotting isn't always cause for concern. About 25% of pregnancies have light implantation bleeding around this time, usually just a pink or brown tint when you wipe. But always mention any bleeding to your provider.

• Skip hot baths. Your baby is most vulnerable to temperature changes right now as neural tubes form.
• Check your meds. Many providers recommend stopping NSAIDs (like ibuprofen) now, ask about safer alternatives.
• No anxiety over caffeine yet. Current guidelines say under 200mg per day is likely fine (that's about one strong cup of coffee).

What might happen

Your first pregnancy test might happen now. The earliest at-home tests can detect pregnancy about 10-14 days after conception. If it's negative but your period doesn't arrive, test again in a few days.

Some providers will schedule early. If you've had previous losses or high-risk factors, your doctor might want to confirm the pregnancy with blood tests (quantitative hCG and progesterone levels) right away.

Your embryo is dividing at lightning speed. Right now, your tiny poppy seed is organizing into three distinct layers that will become every organ and tissue. The outermost layer will form skin, hair, nails, and the nervous system.

At four weeks, you aren't just growing a baby, you're building an entire life support system from scratch.

• Start or continue prenatal vitamins with at least 400mcg folic acid
• Track any unusual symptoms in a pregnancy journal or app
• Schedule your first prenatal appointment if you haven't already