Week-by-Week Pregnancy Development Guide
By week 16, expect your baby’s first movements and kicks; by week 22, prepare for stretch marks; and by week 37, get ready to welcome your child into the world.
Anticipate your baby’s initial movements by the 16th week, the appearance of stretch marks around week 22, and prepare for delivery starting from week 37.
Table of Contents- Weeks 1–4 of Pregnancy
- Weeks 5–8 of Pregnancy
- Weeks 9–12 of Pregnancy
- Weeks 13–16 of Pregnancy
- Weeks 17–20 of Pregnancy
- Weeks 21–24 of Pregnancy
- Weeks 25–28 of Pregnancy
- Weeks 29–32 of Pregnancy
- Weeks 33–36 of Pregnancy
- Weeks 37–40 of Pregnancy
Weeks 1–4 of Pregnancy
Weeks 1–2
Conception has not yet occurred. During this phase, the future mother’s body prepares for pregnancy by developing an egg that will travel through the fallopian tube upon ovulation. Obstetricians count this preparation period as part of the overall pregnancy term, known as the obstetric age. Unlike the actual gestational age, it starts from the first day of the last menstrual period and spans approximately 9 calendar months or 10 obstetric months (each consisting of 4 weeks).
For a detailed explanation of the difference between actual and obstetric pregnancy terms, refer to specialized resources.
Key Information
The first day of your last menstrual cycle allows you to estimate your baby’s due date if conception occurs during this cycle. This is done using Naegele's Rule, which states that there are exactly 280 days, or 40 obstetric weeks, from the first day of the last period to childbirth.
Follow these three steps to calculate your due date:
- Record the first day of your last menstrual period.
- Subtract three calendar months from that date.
- Add one year and seven days to the result.
For example, if your last period began on October 15, 2020, subtracting three months gives July 15, 2020, and adding one year and seven days results in a due date of July 22, 2021.
Week 3
This week marks the true beginning of pregnancy: ovulation occurs, releasing a mature egg from the ovary to meet sperm. If fertilization is successful, the fertilized egg implants into the uterine lining, initiating your baby’s development.
Key Information
For conception to occur, intercourse should take place during ovulation or within 1–2 days prior. Ovulation, the release and journey of the egg through the fallopian tube, lasts only 12–24 hours. Accurate timing increases the likelihood of pregnancy.
Week 4
The embryo grows and develops within the uterine lining but is still too small to be easily detected by ultrasound. It consists of three functional layers enclosed in a membrane:
- Inner layer (endoderm) will form the respiratory and digestive systems;
- Middle layer (mesoderm) will develop into the heart, blood vessels, muscles, and bones;
- Outer layer (ectoderm) will become the brain, nervous system, eyes, skin, teeth, and nails.
The membrane will become the placenta, providing nutrition from mother to baby. Early on, the embryo extends tiny villi from the future placenta to anchor itself and absorb nutrients directly from maternal blood.
Key Information
Women may begin to notice early pregnancy signs this week, such as a missed period. Home pregnancy tests are likely to show positive results with two lines.
Weeks 5–8 of Pregnancy
Week 5
The embryo reaches about 2 mm in size, comparable to a sesame seed. The heart and blood vessels have begun forming, and blood circulation has started, although these are difficult to detect via ultrasound at this stage.
Key Information
The neural tube, which will develop into the brain and spinal cord, begins forming this week. Defects in the tail end can lead to spinal malformations, while issues at the head end may cause severe brain and skull abnormalities. To reduce these risks, doctors strongly recommend starting folic acid supplements by week 5 or as soon as pregnancy is confirmed. Consult your gynecologist for dosage and duration.
Week 6
The embryo is curved, with a tail-like structure, resembling a tiny tadpole about the size of a lentil. The heart is still forming but already beats at nearly twice the adult rate, up to 160 beats per minute.
A transvaginal ultrasound this week may reveal the baby’s heartbeat. Limb buds appear as small protrusions where arms and legs will develop.
Key Information
Morning sickness often begins around this time, causing nausea, aversions to certain smells and foods, and occasional vomiting. This usually subsides within a few weeks.
Week 7
The embryo grows to about 10 mm, roughly the size of a large blueberry or blackcurrant. It lacks legs, so this measurement is from head to rump. The head is the largest part, with rapid brain growth and nerve cell development. Continuing folic acid and any other prescribed vitamins is essential.
Key Information
The uterus has expanded to the size of a medium lemon, pressing on internal organs including the bladder, which may cause increased urination. This typically lasts several weeks until the body adapts.
Week 8
The baby reaches approximately 2 cm in length and is now officially called a fetus instead of an embryo.
Hands and feet develop further, with tiny webbed fingers becoming distinguishable.
Key Information
The mother’s body begins preparing for milk production, often resulting in breast enlargement. A bra size increase of 1–2 sizes may be necessary.
Weeks 9–12 of Pregnancy
Week 9
The fetus grows to about 2.5 cm, similar to a grape. Recognizable human features emerge, including arms, legs, elbows, and knees. Internal organs develop rapidly; the heart divides into four chambers, and tooth buds appear.
Key Information
Early pregnancy discomforts such as bloating, breast tenderness, nausea, and insomnia are common. However, persistent lower back pain, facial swelling, or constant itching should prompt immediate consultation with your healthcare provider.
Week 10
The heart is fully formed and beats around 180 times per minute. The fetus becomes more active, making small movements visible on ultrasound.
Size is comparable to an apricot, with the head occupying nearly half the length due to rapid brain development.
Key Information
Between weeks 10 and 12, a first trimester screening including ultrasound and other tests is recommended to detect chromosomal abnormalities like Down syndrome or major developmental defects.
Week 11
The embryonic period ends, and the fetal period begins. All body parts and organs are formed, many already functioning.
The fetus moves actively, performing jumps and rolls, though the mother cannot yet feel these movements.
Key Information
External genitalia start developing, but it is still too early to determine the baby’s sex accurately. Wait 2–3 more weeks for reliable ultrasound results.
Week 12
The placenta is fully developed, and the fetus reaches the size of a medium mandarin.
Key Information
The uterus has grown enough to be felt through the abdominal wall. Mild abdominal pains or cramps from ligament stretching are common but usually harmless. If pain persists, worsens, or is accompanied by bleeding, seek medical attention promptly.
Weeks 13–16 of Pregnancy
Week 13
The first trimester ends and the second begins. The fetus is about the size of a pea pod and weighs approximately 25 grams.
Key Information
If morning sickness was present, it likely subsides by now. The uterus shifts upward, relieving pressure on the bladder, reducing frequent urination.
Week 14
Ultrasound shows a tiny human about 8.5 cm long. The fetus moves, makes facial expressions, and occasionally swallows amniotic fluid. The kidneys start functioning, and urine is excreted back into the amniotic sac.
Key Information
From this week, expect steady weight gain of about 1 pound (450 grams) per week. Average pregnancy weight gain ranges between 22 and 27 pounds (10–12.5 kg). Balanced nutrition and physical activity help maintain healthy weight gain.
Week 15
The fetus begins to hear, primarily detecting the mother’s heartbeat, digestive sounds, and voice. External sounds are also perceived, though muffled.
Taste buds and eyes start functioning, and the baby responds to bright light on the mother’s abdomen.
Key Information
Many women experience increased sexual desire at this stage. If the pregnancy is healthy and without complications, intimacy can continue safely until delivery.
Week 16
Some mothers may start feeling their baby’s first movements this week. Although the fetus has been active for weeks, it now has enough size—comparable to an avocado—to be felt.
Key Information
Don’t worry if you don’t feel movements yet or if the baby was active one day and quiet the next. The fetus is still small, and not all movements are perceptible.
Weeks 17–20 of Pregnancy
Week 17
The fetus reaches the size of a turnip, weighing approximately 150 grams. The skeleton begins transforming from soft cartilage to bone, and sweat glands develop.
Key Information
As your baby grows, their nutritional needs increase. Ensure you maintain good blood circulation and receive all essential vitamins, minerals, and trace elements. Consult your gynecologist for personalized dietary advice or vitamin supplements.
Also, avoid sleeping on your back during pregnancy, as the expanding uterus can compress major blood vessels, reducing blood flow to your baby.
Week 18
If you haven’t already, now is the time to determine the baby’s sex, as genitalia are fully formed and clearly visible.
Key Information
Blood volume increases significantly to support both mother and baby, causing slight drops in blood pressure. This may lead to dizziness, especially when standing up quickly from sitting or lying down.
Week 19
The baby’s unique fingerprints are fully formed. The body is coated with a special substance that helps retain heat, as the fetus lacks its own fat layer under the skin.
Key Information
Elevated estrogen levels may cause pigmentation changes on the mother’s face, such as melasma, and darkening of freckles, nipples, and the line from the navel to the pubic bone. These usually fade after delivery.
Week 20
Pregnancy’s midpoint, or 'equator,' is reached. The baby’s legs have fully straightened, so height is now measured from head to heel, similar to adults. The fetus is about 25 cm long, roughly the size of a banana.
Key Information
A routine ultrasound is typically scheduled this week, offering parents their first detailed view of the baby’s face, hiccups, and kicks.
Weeks 21–24 of Pregnancy
Week 21
The fetus weighs around 350 grams and measures about 27 cm. It is now heavier than the placenta. The body is covered with a fine, soft down called lanugo, which may help regulate body temperature. Lanugo disappears before birth.
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