Week 4
Your baby is the size of a Poppy seed
Baby Size
Poppy seed
Length
0.08 in (2 mm)
Weight
Less than 0.04 oz
Trimester
First Trimester
Your Baby at Week 4
Your baby is now an embryo, and the most critical development is underway. The heart — still simple and tube-like — begins to beat this week.
Key milestone: Your baby's heart begins to beat. The neural tube (future brain and spine) forms.
Your Symptoms
Missed period. Fatigue, breast tenderness, nausea may begin.
Tip for This Week
Take a home pregnancy test if you haven't already — hCG levels are now detectable.
Your Body at Week 4
During the first trimester, your body is working overtime to build the placenta and support rapid embryonic development. Hormones like hCG, estrogen, and progesterone surge, causing many of the symptoms you're experiencing. Your blood volume is already increasing, and your uterus is beginning to expand — even if you can't see it yet.
Your baby at week 4 is the size of a Poppy seed, measuring about 0.08 in (2 mm) and weighing roughly Less than 0.04 oz. The focus is on forming all major organ systems that will sustain your baby through the rest of pregnancy and beyond.
Your Baby's Development This Week
Your baby is now an embryo, and the most critical development is underway. The heart — still simple and tube-like — begins to beat this week. The key milestone this week is that your baby's heart begins to beat. the neural tube (future brain and spine) forms.
The first trimester is a period of incredible transformation. From a single cell to a fully formed fetus in just 12 weeks — your baby's development is happening at an astonishing pace. Each week brings new organs, new abilities, and new reasons to marvel at the process.
This week's voice theme
“A heartbeat in the dark — the very first rhythm of you.”
Imagine hearing this every morning — a voice note from your baby that evolves week by week.
Hear your baby's voice →Frequently Asked Questions
When does a baby's heart start beating?
The heart begins beating around week 4 of pregnancy, often before many women even know they're pregnant.
How accurate is a pregnancy test at week 4?
Very accurate. By week 4, hCG levels are typically high enough for most home pregnancy tests to detect.