Those two pink lines. This is the week most women discover they are pregnant as their period fails to arrive. Deep inside, your embryo is no longer just a ball of cells — it is organizing itself into three distinct biological layers that will form every organ in the human body.
🍼 What's Happening With Your Baby
Your embryo is the size of a poppyseed (roughly 2mm). It is currently a 'trilaminar disc' — a flat, pancake-like structure made of three layers: the endoderm (inner), mesoderm (middle), and ectoderm (outer).
The inner layer will form the lungs, liver, and digestive system. The middle layer will become the bones, muscles, kidneys, and heart. The outer layer will form the nervous system, skin, and eyes.
Because the placenta is still in its infancy and cannot yet transfer nutrients efficiently, your baby is currently surviving on a tiny structure called the yolk sac. It provides all nourishment and produces the embryo's first red blood cells until the placenta fully takes over around week 10.
🤰 What You're Feeling This Week
Your period is officially late. This is the hallmark symptom of week 4, driven by the fact that your corpus luteum (the remnant of the follicle that released the egg) is pumping out massive amounts of progesterone to keep the uterine lining intact.
You may feel distinctly 'PMS-like' symptoms — bloating, mood swings, and mild cramping as your uterus begins to stretch. Many women assume their period is just around the corner, right up until they take a test.
Your breasts may feel incredibly sore or heavy, and the areolas (the dark circles around your nipples) may begin to darken and widen slightly. Fatigue is likely setting in hard as your metabolic rate skyrockets.
The Neural Tube Folds
A crucial groove is folding inward along the back of the embryo right now. This is the neural tube, and within weeks, it will zip closed to form your baby's brain and spinal cord.
✅ What To Do This Week
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Call your doctor to schedule — as soon as you get a positive test, call your OB-GYN. They likely won't see you until week 8, but getting on the schedule early is vital.
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Check your medications — review all over-the-counter and prescription medications with a pharmacist or doctor immediately. Many common drugs (like acne medications or high-dose retinol) are teratogenic (harmful to embryos).
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Avoid hot tubs and saunas — raising your core body temperature above 102.2°F (39°C) during this critical phase of neural tube closure significantly increases the risk of birth defects.
"The shock of the positive test. The sudden realization that you are two people. Your body is doing the heaviest lifting of the entire pregnancy right now, laying the cellular foundation for a human life. Rest."
— A Note From Your Body, Week 4