Week 4: Implantation and the start of pregnancy
The fourth week is the time of embryo implantation into the uterus and the beginning of placenta formation.
Baby size: Popcorn kernel (0.2-0.4 mm)
Baby development
From blastocyst to embryo
After successful implantation, your future baby, the size of a popcorn kernel, is officially called an embryo.
It consists of three germ layers, from which during the following weeks all organs and systems will develop:
- Ectoderm (outer layer): it will form the nervous system, brain, skin, hair and eyes.
- Mesoderm (middle layer): it will give rise to the heart, circulatory system, bones, muscles and kidneys.
- Endoderm (inner layer): it will form the lungs, liver, intestine and thyroid gland.
Also now the amniotic sac and the yolk sac are actively forming. The latter temporarily provides the embryo with nutrients until the placenta takes over this function.
This is a fundamental construction stage. Although the embryo is tiny, its "blueprint" already contains all the information about the future person.
Learn more about the amazing growth stages in our general section "Baby Development".
Changes for mom
First "bells" of pregnancy
Thanks to the rise in hCG and progesterone levels you may start noticing the first, barely perceptible pregnancy symptoms. Or you may notice nothing, and that is also absolutely normal.
What you might feel:
- Missed period. This is the most obvious and reliable sign.
- Increased breast sensitivity. They may become painful, swollen, like before a period, but more intense.
- Fatigue. Suddenly you feel as if you ran a marathon, even though you just went to the store.
- Bloating. Jeans may feel tighter not because of uterine growth, but due to hormone action.
- Light cramps. Some women feel a pulling pain in the lower abdomen similar to pre‑menstrual.
Remember that the main indicator is the missed period, the importance of tracking which we wrote about here: "How to track your cycle".
Overview
Week 4: Day X and the first signs
Week four is the week you have been waiting for with hope and excitement. This is when you can get the first confirmation of pregnancy!
The key event of this week is implantation. The tiny blastocyst that traveled to the uterus finally attaches to its wall. This triggers a cascade of hormonal changes.
Your body starts producing human chorionic gonadotropin (ХГЛ) — the same "pregnancy hormone" that home tests react to.
This is the moment when your pregnancy becomes official. The first day of your missed period is the loudest signal that a new life is developing inside you.
The start of this journey is always something special. We share our thoughts on this start in our first post.
Tips for mom
Two lines! What next?
If you took a test and it’s positive — congratulations! This is an incredibly exciting moment. Here’s what you should do now.
Your first steps:
- Take the test correctly. It’s best to do the test in the morning, using the first urine — it has the highest hCG concentration.
- Make an appointment with a doctor. Call your gynecologist to confirm the pregnancy and schedule the first visit. Usually it takes place between weeks 6 and 10.
- Continue taking care of yourself. Healthy nutrition, taking vitamins and quitting harmful habits are now more important than ever.
- Allow yourself all emotions. Joy, fear, uncertainty, excitement — all normal. Give yourself time to absorb the news.
At this stage a balanced diet is especially important. You’ll find our advice in the "Nutrition" section.
Tips for partner
Moment of truth: Your reaction matters
This is the week when you can hear the long‑awaited news. Your reaction, support and involvement now lay the foundation for the next 9 months.
How to be the best support:
- Be present at the moment of the test. Experience this excitement together, regardless of the result.
- Celebrate openly. If the test is positive, your emotions and happiness are the best gift for her.
- Take the initiative. Suggest: "Let me call and book you an appointment with the doctor." This shows your care.
- Be patient with her fatigue and mood. "Hormonal swings" are a reality. Approach them with understanding.
This news changes everything. Strengthen your bond and learn to overcome challenges together with tips from our "Relationships" section.
Checklist
Action plan for the "Two lines" week
A clear list will help you manage emotions and take the first important steps.
- ✅ Buy a quality pregnancy test. Or even two, to be sure.
- 🌅 Do the test in the morning, on the day of the expected missed period or later.
- 👩⚕️ Call the doctor and schedule the first appointment.
- 💊 Check your prenatal vitamins. Don’t miss a single day!
- ❤️ Celebrate this event together. This is your special day.
- 🤫 Discuss when and how to tell the news to close ones. Maybe wait a little.
- 📖 Get inspired for the new stage, by rereading our introductory post.