Your Baby's Development
Four weeks into your pregnancy, your baby (now an embryo) consists of two layers of cells — the epiblast
and the hypoblast. These eventually develop into all of your baby's organs and body
Two other structures that develop now are the amnion and the yolk sac. The amnion,
filled with amniotic fluid, will surround and protect the growing embryo. The yolk
sac will produce blood and help to nourish the embryo until the placenta takes over
The embryo continues to implant in your uterus, burying itself deep within
the endometrium. Some women have slight cramping and spotting during this week while
implantation happens. They might mistake this for a period, especially because this
is around the time their monthly period was due.
Once implanted, the embryo starts to make a hormone called human chorionic gonadotropin
(hCG), which helps to maintain the lining of the uterus. It also sends a signal to
the ovary to stop releasing an egg each month, which stops your monthly periods.
hCG is the hormone that's measured in pregnancy tests. This week, a pregnancy test
probably could detect your pregnancy. hCG also causes the symptoms of pregnancy, which
might start now. Tiredness, tingling or aching breasts, or nausea also can happen
with premenstrual syndrome (PMS). But by the end of this week, your period won't happen.
Instead, your pregnancy is under way!
PREGNANCY CALENDAR: A week-by-week
- Trimester 1
- Trimester 2
- Trimester 3