The First Trimester of Pregnancy: What to Expect

Morning sickness, fatigue, and more - what to expect during the first trimester.

first trimester of pregnancy

tali ditye author mommyhood101  By: Tali Ditye, Ph.D., Co-founder
  Updated: April 28, 2023

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Each day, week, and month of pregnancy will bring surprising changes to your body and mind. Some of these changes will be amazing, and some will make you nervous Nelly!

To help you reduce your uncertainty and anticipate the amazing changes that will happen to your body, below we describe what to expect during the first trimester of pregnancy.

We discuss your baby's development and what you might be feeling and seeing along the way. From your last period to morning sickness and your first ultrasound, we've got you covered!

Note: Every woman will experience pregnancy differently. You might experience some of these symptoms very strongly, and others might be a blip on the radar. Still, others might not notice any symptoms at all! That's completely normal because every woman's body is unique, responding in its own way to the hormonal, metabolic, and physiological changes that occur during pregnancy. If you are ever uncertain, contact your doctor.

Pregnancy: Important Definitions

Before we describe what happens during pregnancy, let's cover some basic terms that are important to know, especially when communicating with your doctor.

What is Gestational Age? When you get pregnant, the development clock starts ticking, but not in a super intuitive way. Instead of talking about the fetal age, most doctors talk about gestational age, which is the age of the fetus since the first day of your last period. It's a little strange because this means that your first two weeks of pregnancy happen before your baby is even conceived. 

What is Fetal Age? This is the more intuitive term because it measures the actual age of the developing fetus. Fetal age begins at conception (technically, implantation) and continues until birth.

Most people refer to gestational age rather than fetal age, but if you need to convert between the two, you can basically just subtract or add 2 weeks. So if your gestational age is 6 weeks, your fetal age is about 4 weeks. If your fetal age is 6 weeks, your gestational age is about 8 weeks.

nestig cloud crib

What are Trimesters? Most people talk about trimesters as three-month phases of fetal development. The truth is that each trimester varies in duration between about 12-14 weeks long. The first trimester of pregnancy starts from the first day of your last period, and extends 13 weeks. The second trimester extends for the next 14 weeks, through week 27. Finally, the third trimester extends from week 28 through week 40+.

To simplify, most people talk about trimesters as three-month phases of pregnancy, with the first trimester being months 1-3, the second-trimester being months 4-6, and the third being months 7-9.

How Long is Pregnancy? From the first day of your last period, pregnancies last 283 days on average. If you divide that by 7 days in a week, you get about 40 weeks. That's why most people talk about the 40 weeks of pregnancy. In reality, most pregnancies last between 38-42 weeks.

The First Trimester: From Period to Fetus

The first trimester brings you all the way from the start of your last period, through intercourse and implantation, to the development of a fetus. These phases carry you through roughly the first three months of your pregnancy, called the first trimester.

month 1 of first trimester of pregnancy

Month 1 Development

All the way from having your period to an embryo developing in your uterus, the first month of pregnancy is huge!

Your first month of pregnancy begins on the first day of your last period and extends for 4 weeks. During this time, a lot of important things occur, starting with your period and ending with an embryo that will slowly become your baby over the next 8 months!

Here's how the first month works.

Regular Period & Follicular Phase. Starting on the first day of your period, we call this the follicular phase because it is when a follicle begins to grow an egg. Assuming you have a normal period, a couple of days later your body will enter what we call the fertile window, which leads up to (and continues past) ovulation. You're most likely to ovulate following a regular period. If your period is not on schedule or is abnormally short or long, it might affect whether you ovulate. 

Fertile Window. Starting about 8-9 days after the first day of your period, your body will enter the fertile window. This means that you're becoming increasingly likely to become pregnant as you approach ovulation. The most fertile days of this time are the five days preceding ovulation and the day of ovulation (Ref 1).

The likelihood of fertilizing an egg increases during those six fertile days, peaking on the day of ovulation (to about 35% odds of fertilizing an egg). You can also fertilize an egg for a day or two after ovulation. This is because sperm can stay alive in a woman's body for up to about 5 days, waiting patiently for the egg to arrive, and it takes a few days for the egg to travel down the fallopian tubes (during which there is a day or two when it can still be successfully fertilized).

pregnancy first trimester period ovulation fertilization implantation

Ovulation. After the follicle produces a mature egg, it will break through the ovarian wall and start traveling down the fallopian tubes, beginning what is called the luteal phase. Typically, there is one egg released per month. This is the window of peak fertility when you are most likely to become pregnant.

How will you know when you're ovulating? One way is to simply count the days since the first day of your period; about 14 days after is when you're most likely to ovulate. Another way is to purchase a digital thermometer and track your body temperature.

The day before you ovulate, you will probably notice your body temperature (called your basal body temperature; BBT) drop by up to half a degree (F). The next day, you might notice it spike up about a degree when you ovulate. Many women track their BBT for a few months to get an understanding of what to expect in terms of temperature change. See the image above for a typical pattern of basal body temperature, and how it relates to ovulation and your fertile window.

Intercourse. Most fertility experts agree that intercourse should occur at least once a day for the three days before ovulation, during ovulation, and two days after ovulation. There's really no secret sauce here, other than the semen itself!

Some believe that elevating the hips during sex will allow gravity to help the sperm reach the egg, and some believe it doesn't change the odds. There is some evidence that heat can reduce the count and activity level of sperm, and lubricants can reduce the likelihood of sperm reaching an egg. So try to keep things natural, and cool, and do like the bunnies do.

pregnancy first trimester from egg to implantation

Fertilization: If a sperm successfully finds an egg and penetrates the wall of the egg, you have fertilization! This produces what is called a zygote, which will start its journey down your fallopian tube toward your uterus. 

Implantation: If a sperm successfully fertilizes the egg, the zygote begins to grow and multiply as it travels down the fallopian tube toward the uterus. Along the way, it turns into what's called a blastocyst. Between 7-9 days after the egg is fertilized, the blastocyst will implant itself into the wall of the uterus.

Embryo Development: Once the blastocyst implants into the wall of the uterus, it splits into two parts - these two parts become the embryo and the placenta. The embryo eventually becomes your baby, and the placenta is your baby's lifeline, providing vital nutrients and oxygen to the developing fetus, and removing waste from the blood.

By the end of the first month, the embryo is now about the size of a sunflower seed (about 1/4" long).  

Month 1: Pregnancy Symptoms          

It is rare to develop any pregnancy symptoms during the first month of your pregnancy. Remember that the first two weeks of the month are your period and then the time leading up to ovulation, so other than making sure you have intercourse during the appropriate time, it's pretty much business as usual.

Once implantation occurs, some mild symptoms can occur: 

Mild Cramping: During implantation, some women report mild stomach discomfort and cramping, which is completely normal and can be a sign that implantation is happening.

Implantation Spotting: Many women also report some mild spotting after ovulation, happening right around the time of your missed period. This is completely normal and considered a sign that implantation may have occurred.

Many women are uncertain whether they are having their period or implantation bleeding. However, implantation spotting should be irregular, low quantity, thinner than your period, either light pink or brown (not red), and should only last a day or two. As detailed in the below image.  

pregnancy implantation bleeding spotting versus period

Metallic Taste in Mouth: Does it taste like you put a couple of pennies in your mouth, or you're chewing on tin foil? Many moms-to-be report a metallic taste during the first trimester.

Nobody knows the exact reason why this occurs (most people suggest it's hormones), but it can become really tough if you're also experiencing nausea or morning sickness.

A tip is to drink cold beverages, especially ones with citric acid (lemonade, orange juice), eat foods with vinegar (pickles, anyone!?), or eat something sour (Ref 2). Most moms report that this helps.

month 2 of first trimester of pregnancy

Month 2 Development

During weeks 5-8 of pregnancy, the embryo will develop from about the size of a sesame seed, to about the size of a large bean (about 1" long).

Most vital organ development occurs during this second month of pregnancy, including the heart, and the eyes and limbs (and little webbed hands and feet!) are emerging. Little bones too! 

Heart Beat: During the second month of pregnancy, the heart will be well-developed and beating steadily.

During weeks 5-6, the fetal heart rate will be around 80-90 beats per minute (bpm), and during weeks 7-8, it will speed up to around 130-170 bpm (Ref 3).

fetal heart rate by week of pregnancy

First Ultrasound: Your obstetrician-gynecologist (OB/GYN) might schedule your first ultrasound around 6-8 weeks of gestational age. No guarantees, however, as many practitioners will only schedule such an early ultrasound for high-risk pregnancies.

During the first ultrasound, an ultrasound technician will be able to hear the heartbeat of the fetus and determine a more accurate gestational age and the due date for your baby. This is done by using something like the chart above.

To see the embryo and hear the heartbeat, the technician will likely need to use what is called a transvaginal ultrasound, where the ultrasound wand is placed inside the vagina to provide a close look at the embryo, placenta, and uterus..

Your ultrasound technician will usually turn up the volume and let you listen to the heartbeat. This will likely be the first time that you will realize you have a human being growing inside of you, with its own heartbeat! Don't be surprised if this brings you to tears!

If you have this early first ultrasound, the technician will also be able to tell if you're having multiples (twins, triplets, etc!), and if you're having an ectopic pregnancy. While rare (about 1% of all pregnancies), an ectopic pregnancy occurs when implantation happens anywhere other than the uterus (usually in the fallopian tubes), and is very dangerous.

Blood Draw and DNA Screening: Your OB/GYN may do a blood draw to test your iron levels and screen for any diseases (like HIV). They may also suggest a non-invasive (like a cheek swab) DNA screening for you and/or your partner, especially if you are at high risk for inherited disorders. That might not happen until the third month (about 9+ weeks).

If your screening places you in a high-risk category, this can lead to additional (and more invasive) testing later in pregnancy, to rule out disorders like Tay-Sachs and cystic fibrosis.

Your OB/GYN may also request a urine test, to rule out infection.

Umbilical Cord: This is when the umbilical cord begins to form. The umbilical cord connects your embryo to the developing placenta, providing a lifeline for delivering nutrients and oxygen to your baby, and helping eliminate waste from your baby's bloodstream.

Genitals - Penis or Vagina: By the end of the second month, the embryo is beginning to develop genitals, providing the first visual clues about whether you're having a boy or a girl. They will be way too tiny to see at this point (smaller than a poppy seed!) on an ultrasound. If you're looking to make some guesses about whether you're having a boy or girl, check out our articles about the Ramzi Method, and some funny old wives' tales about guessing gender during pregnancy.    

Month 2: Pregnancy Symptoms          

By the second month of pregnancy, most women report some noticeable symptoms of their pregnancy.

These include:

Nausea and vomiting: This is often referred to as morning sickness, but the reality is that it can come and go at any time of the day. Research shows that 70-80% of pregnant women experience nausea and/or vomiting during one or more trimesters (Ref 4).

Interestingly, research suggests that nausea and vomiting during pregnancy are typically good signs, and are associated with a lower incidence of adverse pregnancy outcomes (Ref 5).

Nausea and vomiting during pregnancy can also exacerbate food aversions, and cause new associations (whether real or not!) between certain foods and sickness. 

Managing morning sickness: A comprehensive review of the literature (Ref 6) examined the safety and efficacy of nine different morning sickness interventions. These included things like acupuncture, ginger, teas, essential oils, vitamins, and anti-emetics.

The authors concluded that there was no strong and compelling evidence for any intervention, especially in the context of potential risks and the lack of long-term studies examining risks to the mother or fetus. 

They did suggest that ginger tends to be more effective than a placebo in a few studies, but overall the data were lacking to make any definitive conclusions.

Ginger teas, lollipops, or dried strips might be worth trying but always ask your doctor first.

Also try to take prenatal vitamins along with a snack or meal, rather than on an empty stomach.

Eat bland snacks, like a banana, saltines, and oatmeal throughout the day, and avoid spicy or greasy foods. Some women also find success in using an acupressure bracelet.

reducing morning sickness tips

Mood Swings: Many women in the second month of pregnancy (and beyond!) will report mood swings and irritability. This is completely normal and is a sign that your body is producing new hormones to support the development of the fetus.

During the second month of pregnancy, your body will experience a spike in human chorionic gonadotropin (hCG), and begin a slow rise of estrogen and progesterone.

These hormonal fluctuations can make you feel a range of emotions, and that is completely normal and is likely to level off during the second and third trimesters.

It's a good point during your pregnancy to take some time for yourself, relax with a good book, and get some low-intensity exercise.

Urge to Urinate: The increase in hCG hormones also causes higher blood flow to your pelvic region, which in turn can make you need to pee more frequently than usual.

If you find that you can't make it through your morning commute without needing to stop and pee, or through the night without getting up to pee, you are one of the many women who experience this symptom.

The good news is that it's completely normal. The bad news is that it might not get better for another month or two.

While you might be tempted to stop drinking as much water as usual, it's important to stay hydrated, even if it means having to scurry off to the bathroom once an hour!

Heartburn, Bloating, & Gas: The first trimester is the time when heartburn and indigestion typically start to rear their ugly head. The bad news is that it might not get better as your pregnancy progresses.

But the good news is that you might learn some strategies that will help you - like avoiding certain foods (spicy, greasy), certain positions (reclining), or certain clothing (tight-waisted clothes).

Breast Soreness: If your breasts feel more tender than usual, look like they're getting bigger, or your nipples are changing in size, color, and shape, that's completely normal during pregnancy. Your body is getting prepared to breastfeed your baby!

Constipation: Many women begin to experience some mild to moderate constipation during their first trimester, right around months 2 or 3. A few days between bowel movements is pretty common at this stage.

One way to help reduce constipation during pregnancy is to increase your consumption of fruits and vegetables, especially those high in fiber. Not only are these full of healthy nutrients and vitamins, they're also great at keeping you hydrated and helping your bowels move.

More sour fruits, like an orange or grapefruit, can also be great at reducing any bad tastes in your mouth and are full of water to help keep you hydrated. Plenty of water, fruits, and veggies, and light exercise will help reduce the annoyance and pain of constipation.

Leukorrhea Discharge: You might notice a thin white discharge coming from your vagina around this time. This is normal, healthy, and a result of increasing estrogen levels and blood flow to your pelvic region. To learn more, check out our article about early pregnancy discharge and leukorrhea

month 3 of first trimester of pregnancy

Month 3 Development

During the third month, technically during week 10 or so, the embryo officially becomes a fetus! It will grow substantially, doubling in size from a large bean (about 1" long) to about the size of a lime (about 2" long).

A Fetus Emerges: The little curled-up bean is starting to open up, with the head straightening out, and limbs extending and moving. This is the point at which the embryo officially becomes a fetus, and starts to look increasingly human.   

Baby Bump: Corresponding changes will be happening to the appearance of your body, and by the end of the first trimester you will probably be noticeably pregnant (at least to yourself!).

The little baby bump will also make it a little harder to fit into your usual clothing, putting you in limbo between what are suddenly tight-fitting clothes, and maternity clothes that you might not be ready for. Try not to wear tight clothing around the waist, to reduce the pressure of bloating and indigestion. 

fetal development stages pregnancy belly growth

Bones and Teeth: Inside, the fetus is starting to look and act like a person. The arms and legs wiggle around, flexing their little muscles. Bones start to develop, and even the first little teeth start to form under the gums.

Even though your baby is growing bones and might start wiggling and kicking, it's far too small for you to feel anything at this point.

Eyes, Knees, and Toes: Your fetus has little ears emerging, their bones and joints are forming, and the little webbed toes and fingers are starting to look more normal.   

Month 3: Pregnancy Symptoms

The Usual Symptoms: All the same symptoms that plagued you during months 1-2 will also be around for month three. This includes constipation, heartburn and indigestion, nausea and vomiting, mood swings, food cravings and aversions, and the need to urinate frequently.

Visible Veins: You might notice thin blue lines starting to appear on your tummy and breasts. These veins are carrying all the new blood supply that's moving into your pelvis, torso, and breasts, and providing nourishment for your developing fetus (and your baby-to-be!).

It is likely that the veins in these areas will become increasingly noticeable as you progress into the second and third trimesters. But don't worry, they are likely to subside once your baby is born and after you wean from breastfeeding.

veins on breasts and stomach pregnancy

Fatigue: Your body is working overtime and you're likely to feel tired throughout the day. If you got an afternoon slump before getting pregnant, it's suddenly more extreme. The same goes for the evening when you might feel like you need to sleep for 12 hours a night.

Sleep is great, and get it whenever you can. A cat nap and then a walk can be refreshing and energizing. While some people will suggest you "sleep now while you can!" not all women find it easy to detach from their lives (and prepare for a baby) and end up getting less sleep than usual.

Between the need to pee every few hours and heartburn when laying down, sleep might become more elusive than ever.

Light-headed: As your circulatory system expands, your body needs to create more blood to fill all the new and wider veins that provide blood (nutrients and oxygen) to your lady parts and developing fetus. Your body will be working overtime to produce blood, and when you're not getting enough you might feel faint or dizzy.

This is pretty common toward the end of the first trimester. We suggest lying down with your feet up, helping to get more blood flowing throughout your entire body. If it continues, contact your doctor.

Headaches: Those pesky hormones, in conjunction with the changes in your circulatory system, might give you some intermittent headaches. Make sure you stay hydrated and ask your doctor about options for pain relief.

Next: The Second Trimester!


In developing this guide, we consulted with physicians, pediatricians, and biologists. Additional resources were gathered from The Office on Women's Health, the Nemours Foundation, American Pregnancy Association, and the Mayo and Cleveland Clinics. Additional scientific resources (Refs 1-5) cited in the article can be found here:

1. Wilcox, A. J., Weinberg, C. R., & Baird, D. D. (1995). Timing of sexual intercourse in relation to ovulation—effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine333(23), 1517-1521. https://www.nejm.org/doi/full/10.1056/NEJM199512073332301

2. Tierson, F. D., Olsen, C. L., & Hook, E. B. (1985). Influence of cravings and aversions on diet in pregnancy. Ecology of Food and Nutrition17(2), 117-129. https://www.tandfonline.com/doi/abs/10.1080/03670244.1985.9990886

3. Knipe, H., & Weerakkody, Y., et al. (2010). Fetal heart rate. Radiopaediahttps://radiopaedia.org/articles/fetal-heart-rate?lang=us

4. Lee, N. M., & Saha, S. (2011). Nausea and vomiting of pregnancy. Gastroenterology Clinics40(2), 309-334. https://www.gastro.theclinics.com/article/S0889-8553(11)00027-6/abstract

5. Tierson, F. D., Olsen, C. L., & Hook, E. B. (1986). Nausea and vomiting of pregnancy and association with pregnancy outcome. American journal of obstetrics and gynecology155(5), 1017-1022. https://www.sciencedirect.com/science/article/abs/pii/0002937886903376

6. Matthews, A., Haas, D. M., O'Mathúna, D. P., & Dowswell, T. (2015). Interventions for nausea and vomiting in early pregnancy. Cochrane Database of Systematic Reviews, (9). Read the article here.

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