Tag Archives: pregnancy timeline

Science of Pregnancy Week Two: Fertility

18 Apr

Your egg, preparing for debut

Girls are born with ~1-2 million eggs in their ovaries. In adulthood, a fraction of these eggs ripen. In a typical menstrual cycle, one egg is launched each month on a great lazy river ride. It’s released from the ovary, swept up in the tendrils of the fallopian tube, and ushered by a gentle tide toward the uterus.

When you had your period last week, your body flushed out an egg that was not fertilized during that adventure. But your body is already at work preparing the next contestant.

If you are trying to conceive, your most fertile period is in this window: the week leading up to, and including, ovulation.  Check an ovulation calendar to identify when you are most likely to become pregnant.

During this time, several hormones are working like greek goddesses in your system, bestowing vital gifts to prepare your egg for its journey: protective shield, snack bags, a safe landing zone— and plenty of sperm-grabber, just in case.

oocyte

Miscarriage happens for a lot of really good reasons.

29 May

Loss is loss. And grief about loss is real.

But when miscarriage happens, it’s usually for a very good reason.

In the first weeks after conception, the genome is trying itself out.

“Can I make organs? If not, I am going to have a hard time on this planet. Let’s try again.”

Every baby that is born represents a spectacular cellular success. A cell that can magic itself into becoming a living being has to accomplish an elaborate trick; as we’ve pointed out before, making organs alone is like getting a Boeing 747 off the ground.

Life is rare and precious, but the possibility of life forming  is also astronomical. It’s why we have so many sperm and so many eggs. The whole system is set up to make life work. And that’s why they made sex fun.

So losing an embryo can be an opportunity to try again, and not a sign of something that is systematically wrong.

There’s a million ways that we form as snowflakes.

There’s a time to go at it again and have some more fun with it.

Pregnancy timeline: 4 to 8 days after conception

5 Aug

Four days after fertilization, a wad of 16 to 32 identical cells (the berry-like “morula”) approaches the uterine cavity.

A week after fertilization, the blastocyst lands, thanks to cellular communication. Image by Tevah Platt.

But in the following day or two, it starts to look more like a dented soccer ball, with an outer ring of cells surrounding an inner cluster. Scientists have an unsexy name for this cell-wad: The blastocyst.

The geographic parsing of inner and outer cells in the blastocyst is exciting because it is the first step toward differentiation (the amazing sorting of cells into different cell types: for example, skin cells, blood cells, eye cells, etc.). The inner cells will later form the embryo, and the other cells will form the placenta.

These stem cells are the ones that are valued by researchers because they haven’t reached a point on the path of development at which they acquire super-specialized characteristics and become committed cell types like eye cells or liver cells. They are no longer “totipotent” (able to become any kind of cell); they are now “pluripotent” (able to become one of many kinds of cell in the future).

At this stage, communication across cells has ramped up with the creation of ion channels, gap junctions and protein channels– all fancy words for doors and windows through which cells can essentially chat with their neighbors.

Molecules that go in and out of these windows are the messages that circulate among the cells, allowing parts of the body to work together.

Seven or eight days after fertilization, the cells of the blastocyst also begin to coordinate with the cells in mom’s body.

For example, the blastocyst receives chemical signals from the lining of the uterus that guide them to a safe spot for landing. In turn, they secrete enzymes that clear the ground for implantation. Cellular cooperation will be critical over the next 9 months, not only for the exchange of nutrients and oxygen between mother and embryo, but for the orchestration of the embryo’s development.

We don’t often think about it, but we know from our everyday actions that our cells work together all the time. Our hands and mouths cooperate every time we eat a hunk of cheese. When Lady Gaga put on a suit made of meat, her eye cells and arm cells had to communicate with the brain cells that made that decision. Ask any person who is quadriplegic and has had his nerve cells cut off and you realize that relationships within the body run everything.

Once we have cells that can communicate, we have cells that know where they are, that can be called upon to make stuff, and that can become certain types of cells.

Safely lodged in the uterus, the ever-dividing cells can turn to the work of organizing themselves, laying out the basic plan of the body, and slowly and gently, beginning the process of differentiation that will give rise to bones and ears and knuckles.

Pregnancy timeline: One to three days after conception

4 Aug

During the first days of pregnancy, cells divide to create duplicates of the original, fertilized egg. As genes become activated, the cells begin to communicate by sending and receiving chemical signals.

Compared to cells that will later allow baby to babble and barf, the first, original cell doesn’t have much to do. Before fertilization, the egg doesn’t need a lot of protein channels, or windows and doors through which it could “chat” with its neighbors. It has only to look for one thing in its environment: The wiggly-tailed sperm. It is set up with the chemical matrix to sense sperm, to help that first wiggler to traverse into its nucleus, and to create a barrier to deflect also-rans in the sperm race.

About a day after conception, the egg divides for the first time, making an exact copy of itself.

Only now that the egg is fertilized and is making its way down the fallopian tube will it develop the chemical recognizers it will need at the end of the week to lodge into the warm wall of the uterus. Creating them any sooner would be a waste of molecular time and energy, like putting wheels on a car that might never be driven.

Making their way womb-ward, the cells that began with the egg divide every 15 hours or so. Two days after fertilization, 2 cells become 4. On day 3, 4 cells become 8.

At about the 8-cell stage, the machinery within each cell starts to click into gear and turn on. The cells have not differentiated into distinct types like eye cells or liver cells; they are at this stage still “totipotent,” or capable of becoming any type of body cell.

But now, in addition to replicating their chromosomes in order to duplicate, they begin to activate genes to create little protein machines or structural scaffolding for the cells. And they begin to produce chemicals that are communicators and recognizers– transmitters and receivers of molecular messages.

Communication between little cells will drive the development of the embryo as a whole.