Introduction to Human Development - Online Tutor, Practice Problems & Exam Prep

Conception occurs when an oocyte is fertilized by a sperm, marking the start of development. The prenatal period, lasting about 40 weeks, is divided into three stages: the preembryonic stage (weeks 1-2), embryonic period (weeks 3-8), and fetal stage (weeks 9-38). Key processes include cleavage, implantation, and organogenesis. The conceptus is referred to as a blastocyst, embryo, and fetus during these stages. Understanding these stages and their timelines is crucial for grasping human development.

Introduction to Human Development

Hey, everyone. And welcome to the human development chapter. In this video, we're just going to go over some basic terminology to kind of get us started. We're going to begin with the term conception. Conception is the moment when an oocyte is fertilized by a sperm cell. You guys remember an oocyte is just a term for an immature egg. So conception marks the beginning of development. Throughout this course, you're going to hear me referring to a conceptus. A conceptus is just a term to refer to the developing offspring. And what's nice about this term is that it can refer to either an embryo or a fetus. As you'll see in an upcoming video, the terms embryo and fetus both imply something about the age of the conceptus, whereas conceptus is a nice broad term that does not imply anything about the development happening. So it's just a nice general term that we can use.

Then finally, it's good to understand how we measure the gestational period, also known as the prenatal period. The prenatal or gestational period is the time during which the development of the conceptus occurs. The period extends from the date of the mother's last menstrual period. So we're looking at the first date of the last menstrual, or most recent menstrual period, which is sometimes abbreviated to LMP, right up until birth. This period is usually about 40 weeks long.

Now, astute viewers may have noticed, like, "Hannah, you just said that conception begins when an oocyte is fertilized by a sperm cell. That's not the same thing as the date of the last menstrual period. What's up with that?" Love that you're asking questions. Fantastic point. It is kind of weird. As you may recall, women typically ovulate about 2 weeks into their cycle. So week 2 is usually the actual date of conception. At the moment of conception, a woman is already considered 2 weeks pregnant. This time from the first day of the period up until week 2, nothing is happening. There is no development occurring. That's just empty space there.

We use the date of the mother's last menstrual period because it's much more noticeable. It is a pretty reliable date that women can report on, and then we can usually estimate that ovulation would take place approximately 2 weeks after that date. But this does mean you have to think about it because that means that from the moment of conception through birth, this period, represented by this yellow arrow, is going to be approximately 38 weeks. The traditional way of measuring pregnancy is a gestational period of 40 weeks long, wherein the first two weeks nothing is happening. The actual development of the conceptus is taking place over approximately a 38-week period.

Just so you know, this course, this chapter is going to be focusing on the 38 weeks of prenatal development, the actual 38 weeks of development from conception until birth. So our day 1 is the moment of conception essentially. But do be aware of this discrepancy. Some textbooks will talk about the dates a little differently. Some professors will use different dates, so be aware of how your textbook and your professor talk about it, but those are the two traditional ways that we think about the prenatal period. Alright. That is our little primer for human development and I will see you guys in the next one. Bye bye.

Introduction to Human Development Example 1

Judy takes a pregnancy test after realizing her period is one day late. When the test comes back positive, she immediately calls her OBGYN to set up an appointment. On the phone, she tells them the date of her last menstrual period and is confused when the doctor refers to her as already being 4 weeks pregnant. If you were the doctor, how would you explain this to Judy? So let's run through these choices and see which one is the best explanation for that.

Choice A reads, conception typically takes place within 1 to 2 days of the menstrual period ending, so using the date of the last period is a pretty accurate marker. And that's definitely incorrect. Right? We know that for conception to take place, ovulation has to occur and ovulation will take place about 2 weeks after the period, not 1 to 2 days after it ends for most women. So, A is definitely not a very good explanation.

Choice B reads that important prenatal events like placenta development actually take place in the time between the menstrual period and conception, and that's also incorrect. We know that there is actually no development taking place during that time at all. We need conception for any development to actually begin. So B is not a good explanation either.

Choice C reads, we often measure pregnancy starting with the date of a woman's last period rather than the date of conception because reporting tends to be more accurate, which means that for 2 of those 4 weeks you weren't even pregnant yet. And that is a fantastic explanation. So we really do, we use that date of the last period because women are just much better at reporting it. Women typically don't know the day of ovulation, they usually know the day of their first, of their period beginning. So C is definitely a great answer.

Then Choice D reads it's impossible to accurately predict the day of conception so it's best to ballpark it and that's not true either. We have a pretty good way to estimate that date using the day of the last menstrual period. Alright, so our answer here is going to be C.

True or False (if false, choose the answer that would correct the statement): Conception is the moment an embryo begins to develop.

• A. True.
• B. False: Conception is the moment a person discovers they are pregnant.
• C. False: Conception is the moment an oocyte is fertilized by a sperm cell.
• D. False: Conception is the act of sexual intercourse.

Timeline of the Prenatal Period

In this video, we're going to go over a timeline of the prenatal period just to begin to introduce some vocabulary as well as talk about some of the major events that we're going to be covering in this chapter. The prenatal or the gestational period is often divided into 3 stages. First up is the preembryonic stage, which spans weeks 1-2 post-conception, and during the majority of this time, we're going to refer to the conceptus as a blastocyst. It is going to have a few different names earlier in development, but for the majority of those 2 weeks, we are going to call it a blastocyst. And you can see what a blastocyst looks like right there. Basically, just a big old pile of cells at this point, still very early in development.

Next up, we have the embryonic period, which spans week 3 through week 8 post-conception, and during that time, we're going to refer to the conceptus as an embryo. And you can see, just in these images, how the embryonic period is a time of very rapid development. So we're going to start that period looking like this as a little trilaminate or 3-layered disc of cells, and then we're going to kind of enter that more like tadpole-like stage and then finally, we're going to end that stage looking a little bit more distinctly human by that point.

Then we are going to get into the fetal stage, which is going to span weeks 9 through approximately 38 or when birth takes place. It could be 39, could be 37, just depends on what that baby wants to do. And during that time, we're going to refer to the conceptus as a fetus. And as you can see from our image here, a fetus looks pretty much like a little baby, albeit a very teeny tiny baby for the majority of those earlier weeks there. So you can see, we kind of have a timeline up along the top here, spanning from week 1 to week 38 of development. And then along the left side, we have this list of events that will be taking place, and these are listed here in the order that we're going to cover them in this chapter. So feel free to refer to this, as a kind of map to let you know where you are in the chapter and what topics are going to be coming up next.

Via these pink lines, we are indicating when these events are taking place. The solid colored pink lines refer to major developmental events happening, and the striped lines refer to some development taking place, but the majority would be occurring wherever we see that solid pink line. We'll begin with fertilization, which of course is the very first event happening in week 1. Cleavage and blastocyst formation are also happening in week 1. We will be talking about implantation, which is the process of the blastocyst embedding itself into the uterine wall, happening during our preembryonic development stage. We'll talk about the development of the extraembryonic membranes or membranes that develop outside of the embryo. That begins in our preembryonic stage, but the majority of that growth happens during our embryonic period. We'll discuss gastrulation, which is the process of creating that little guy right there and happens in the preembryonic and very early embryonic phase. Then we'll discuss organogenesis, the creation of organs, happening during our embryonic phase. We'll talk about placentation, the creation and growth of the placenta that begins very early in development around week 2 and spans pretty much all of the embryonic development. The placenta is usually functional by about 12 weeks, but then it will continue to grow a bit throughout the fetal stage as well. Finally, we'll cover organ development and skeletal growth, which begins in our embryonic period, but the vast majority of that growth, of course, will be more in the fetal stage of development.

That is our timeline of the prenatal period. Again, feel free to refer back to this lesson to see where you are as you go through the chapter and what events are going to be coming up next or what events are happening during which week. See you guys in the next one. Bye-bye.

Introduction to Human Development Example 2

Okay. So this one reads, at 10 weeks, a conceptus is commonly referred to as what? So looking at our answer choices, I know we can immediately rule out 'infant' because we're only going to call them an infant once they're already born, and we can rule out 'blastocyst' for sure because a blastocyst is a very, very early stage. That's like 1 to 2 weeks of development. So we're left with 'embryo' and 'fetus'. Remember, an embryo is how we refer to a conceptus between 3 to 8 weeks of development, and then a fetus is from weeks 9 to approximately 40 or whenever birth takes place. So, a 10-week conceptus is going to be referred to as a fetus, and there you go.

Here’s what students ask on this topic:

What is the difference between an embryo and a fetus?

An embryo and a fetus are terms used to describe different stages of prenatal development. The term 'embryo' is used from week 3 to week 8 post-conception. During this period, the conceptus undergoes rapid development, transforming from a trilaminar disc to a more human-like form. The term 'fetus' is used from week 9 until birth, approximately week 38. During the fetal stage, the conceptus looks more like a tiny baby and undergoes significant growth and organ development. Understanding these stages helps in grasping the timeline and key developmental milestones in human development.

How is the gestational period measured?

The gestational period, also known as the prenatal period, is traditionally measured from the first day of the mother's last menstrual period (LMP) to birth, lasting about 40 weeks. This method includes the first two weeks before conception, as ovulation typically occurs around two weeks into the menstrual cycle. Therefore, actual development from conception to birth spans approximately 38 weeks. This method is used because the LMP is a more noticeable and reliable date for women to report, allowing for a consistent estimation of the gestational period.

What are the three stages of prenatal development?

The three stages of prenatal development are the preembryonic stage, the embryonic period, and the fetal stage. The preembryonic stage spans weeks 1-2 post-conception, during which the conceptus is referred to as a blastocyst. The embryonic period covers weeks 3-8 post-conception, where the conceptus is called an embryo and undergoes rapid development. The fetal stage extends from week 9 to approximately week 38, during which the conceptus is known as a fetus and continues to grow and develop until birth.

What is organogenesis and when does it occur?

Organogenesis is the process of organ formation during prenatal development. It primarily occurs during the embryonic period, spanning weeks 3-8 post-conception. During this time, the three germ layers (ectoderm, mesoderm, and endoderm) differentiate into various tissues and organs. This period is crucial as the foundations for all major organ systems are established. Any disruptions during organogenesis can lead to congenital anomalies, making it a critical phase in human development.

What is the significance of the blastocyst stage in human development?

The blastocyst stage is significant in human development as it occurs during the preembryonic stage, spanning weeks 1-2 post-conception. During this stage, the conceptus is a blastocyst, a structure consisting of a hollow ball of cells. The blastocyst undergoes cleavage and prepares for implantation into the uterine wall. Successful implantation is crucial for the continuation of pregnancy, as it allows the blastocyst to receive nutrients and begin forming the extraembryonic membranes and placenta, essential for supporting the developing embryo.