4 Weeks Pregnant
Pregnancy Week 4 & Stage of Fetal Development

At the end of this week, you will be 4 weeks pregnant (see pregnancy chart). Implantation takes place this week and at the end of pregnancy week 4 you'll be able to take a home pregnancy test to determine if you are pregnant or not.

The 4th week marks the final week of the two week wait. It generally takes about 2 weeks from conception before you can confirm your pregnancy.

The gestational period is defined as 38 weeks plus 2 weeks. That's from conception (fertilization) to birth plus the two weeks before. So the 40 weeks of pregnancy begins 2 weeks before conception or the first day of your LMP (last menstrual period).

In my opinion, this week is one of the most frustrating mostly because there is not much you can do but anxiously wait to pee on a stick (POAS/take a home pregnancy test).

What is happening at 4 weeks pregnant

Well, ovulation and conception (fertilization) has occurred which happens around the beginning of week 3. And your little one has already traveled down the fallopian tube to the uterus.

At the beginning of pregnancy week 4, the embryo is floating around in the uterus. In a couple of days will find a spot to burrow into the endometrium (lining of the uterus) to receive nutrition from the mother.

Implantation generally takes close to a week to complete. The embryo (called a blastocyst at this stage) will dig into the uterine lining. And slowing make its way deeper to establish contact with the mothers blood supply. And finally, will be completely covered by the endometrial lining at 4 weeks pregnant.

Once implantation occurs, you will be officially pregnant but will not know it yet (except maybe by intuition). This is when the embryo starts to secrete a hormone called hCG (human Chorionic Gonadotropin) the pregnancy hormone.

It takes about a week for hCG levels to be high enough to be detected by home pregnancy tests. That's why you have to wait until (about) the end of pregnancy week 4.

Progesterone levels are on the rise and is the dominant hormone in the 4th week of your ovulation cycle (luteal phase). The corpus luteum (the empty follicle that the egg came from) is secreting the hormones (including estrogen) necessary to prepare your body for possible pregnancy.

Once the corpus luteum receives the signal from hCG from the embryo, it will continue to produce hormones for the continued support of your little one.

If implantation does not happen, then the corpus luteum does not receive the signal from hCG from the corpus luteum and will begin to degenerate, producing less and less hormones. It's the drop in progesterone levels that signals your uterus that pregnancy (implantation) did not occur and will begin to break down it's endometrial lining and you will get your period.

That is why it is said that implantation marks when you are officially pregnant because without it pregnancy cannot continue.

There are several reasons why an embryo will not implant, mostly for genetic reasons. Most women will not even know that conception has occurred because she will get a normal period. This happens generally about 30% of the time and is normal and natural. Your body is doing what it needs to do to by eliminating a nonviable pregnancy.

What you can do at 4 weeks pregnant

This is the second week of what is known as the two week wait. There is not much you can actively do at this stage, but wait to take a pregnancy test near/at the end of this week.

You've done the deed on the days leading up to ovulation and are now waiting to find out if it happened.

Many of you, including myself, have found this time to be unbearably frustrating.  Try to relax, easier said than done, I know.

Take this time to maybe think about baby names.  Make notes in your pregnancy journal.  Make a list of the things you'll need for your little one after she's born.  You won't need to run out and get them right away, but this will help take your mind off of the wait.

Work on a hobby that you've been putting off.  Catch up with friends that you haven't seen in awhile.  Read that book you've been meaning to read, but have been putting off.  It won't be long now.  :-) 

Fetal Development at 4 weeks pregnant
Fetal age: 2 weeks

At the end of week 3, the blastocyst differentiated into two groups of cells; trophoblast and embryoblast (or inner cell mass) and has hatched from its protective shell (zona pellucida).

In pregnancy week 4, the blastocyst enters the uterus for implantation. The amniotic cavity and the placenta are starting to form.

The trophoblast cells (which will become the placenta and umbilical cord) that make up the outer layer begins to grow. It's these cells which 'dig' into the endometrial lining during implantation.

At the same time, the embryoblast will differentiate into two more groups of cells. The epiblast (which will become your baby and the amniotic sac) and the hypoblast (which will form the yolk sac).

The yolk sac is what will nourish your baby until the placenta is functioning at which time nourishment will then come from the mother.

From the epiblast, three more layers of cells form (called germ layers). These layers are called endoderm, mesoderm, ectoderm.

Each layer will become different parts of your baby.

The endoderm becomes lungs, liver, pancreas, stomach, bowel, bladder, and thyroid.

The mesoderm becomes skeleton, muscles, connective tissues, heart, blood vessels, and reproductive organs.

The ectoderm becomes the nervous system, brain, skin, hair, nails, nipples, tooth enamel, and lenses of the eyes.

These are cells in their primitive form and mark the beginning of the embryonic stage of development. The embryonic stage is when all the organs are being formed but most will not be complete or functional yet.

The fetal stage of development begins when all organs have been formed even though many still will not be functioning yet.

And, your little embryo is still only about 0.1mm to 0.5mm in diameter at 4 weeks pregnant even though it has gone through much growth (cell division and differentiation).

Now that you are 4 weeks pregnant, your baby has implanted and is growing rapidly. Have you tested yet? BABY DUST!!

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