3 weeks pregnant: baby is a single cell that is already dividing

Your baby this week

Mayo Clinic describes fertilization as the moment sperm and egg unite in one of the fallopian tubes to form a one-celled zygote. The zygote holds 46 chromosomes, 23 from each parent. Those chromosomes set your baby's genetic sex and many inherited traits.

How it happens. Millions of sperm start the journey, and only one enters the egg. Once one does, the egg changes its outer layer so no others can follow. The two sets of genetic material then combine.

What follows, as a rough timeline:

• Within about a day: the zygote divides into two cells.
• Around day 3 to 4: it becomes a solid ball of cells called a morula, which Mayo Clinic and Cleveland Clinic both describe as the next stage.
• Around day 5: it turns into a hollow ball called a blastocyst, with an inner group of cells that will become the embryo and an outer layer that will become the placenta.

All the while, the ball of cells drifts through the fallopian tube toward the uterus, helped by tiny hair-like structures and gentle muscle contractions. It does not grow in size yet. The cells divide into smaller ones.

How twins happen

Twins form in two ways. Fraternal twins happen when two separate eggs are fertilized by two sperm, so they are as genetically alike as any siblings. Identical twins form when a single fertilized egg splits in two early on, so they share the same DNA. The timing of the split affects how much they share, such as a placenta.

Neither type is something you can detect at 3 weeks. An ultrasound in the first trimester shows how many babies you are expecting.

Your body this week

You will not feel fertilization or the first cell divisions. The lining of your uterus is thick and rich, waiting for the blastocyst to attach, and a hormone called progesterone is keeping it that way.

The waiting period between ovulation and the day you could take a test is often called the two-week wait. It can feel long, especially when you notice every twinge.

Why early symptom-spotting is unreliable

After ovulation, your body makes progesterone whether or not you conceive. That is why so many of the things that feel like early pregnancy also feel like a period on its way.

Things that can appear in either case:

• Tender breasts. Progesterone and estrogen affect breast tissue.
• Mild cramping or a heavy feeling. The uterus is preparing for a lining shed or an implantation.
• Tiredness. Progesterone has a mildly sedating effect.
• Bloating and mood changes. Also typical in the days before a period.

The NHS says that early pregnancy signs usually appear around weeks 4 to 6. A missed period is the most reliable early clue, and a test after that is the reliable way to know.

Habits to keep during the wait

Once you may be pregnant, the following matter even before you know for certain:

• Keep taking folic acid. Most national advice is 400 micrograms daily through the first 10 to 12 weeks, and some countries advise longer.
• Avoid alcohol. Canada's guidance says no amount has been shown to be safe.
• Check medicines and supplements. Some, including anything with vitamin A (retinol) and cod liver oil, are not advised in pregnancy, according to the NHS. Do not stop prescribed medicines without asking your provider.
• Practise food safety. Skip raw or undercooked meat and fish, and unpasteurized dairy.
• Stay active. Walking, swimming and yoga are all fine.