Quick Answer: The 4th month of pregnancy (weeks 13 to 16) is when your belly transitions from "is she or isn't she" to a definite bump. Your uterus rises above the pelvis, the baby grows from about 7 to 15 centimetres, and skin changes like the linea nigra and early stretch marks may appear. This is also when many women feel their best. According to a 2015 systematic review in Sleep Health (Radwan et al.), medium-firm surfaces produced the best outcomes for sleep quality, which is useful guidance for setting up the bedroom ahead of the third trimester.
In This Guide
Reading Time: 9 minutes
Welcome to the 4th month. If the first trimester felt like surviving, the second trimester often feels like arriving. Nausea is fading. Energy is creeping back. And your belly, which stubbornly refused to look pregnant for weeks, has finally decided to announce itself.
This month is about transition. Your body is shifting gears from the intense organ-building phase to a growth phase. The baby is growing rapidly, your skin is stretching to keep up, and you are probably noticing changes you did not read about in any app.
Weeks 13 to 16: What Happens Each Week
Unlike the first trimester when changes were mostly invisible, the 4th month brings weekly milestones you can actually see and feel.
Week 13: The Second Trimester Begins
Your baby is about 7.5 centimetres long, roughly the size of a peach. The vocal cords are forming. The intestines, which had been developing in the umbilical cord, are moving into the abdomen where they belong. Fingerprints are starting to form on those tiny fingertips.
For you, the relief of reaching the second trimester is real. Miscarriage risk drops significantly after week 12. Many couples choose this week to share the news more widely. Physically, you may notice your waistline thickening even if the bump is not fully round yet.
Week 14: Energy Returns
The baby is about 9 centimetres, the size of a lemon. Facial muscles are developing, which means the baby is practising expressions like squinting and frowning. The thyroid gland begins producing hormones.
This is often the week women describe feeling "normal again" after the exhaustion and nausea of the first trimester. Appetite typically returns, sometimes forcefully. If you have been struggling to eat well, this is your window to focus on nutrition.
Week 15: Movement Begins
About 10 centimetres now, roughly the size of an apple. The skeleton is visible on ultrasound. Legs are growing longer than the arms. The baby is moving actively, somersaulting, stretching, hiccupping, though most first-time mothers will not feel it for another few weeks.
Your belly is starting to look pregnant rather than just bloated. The uterus is halfway between the pubic bone and the navel. Some women feel the first subtle flutterings this week, especially if they have been pregnant before.
Week 16: The Bump Is Real
The baby is about 12 to 15 centimetres, roughly the size of an avocado, and weighs about 170 grams. The eyes are sensitive to light, though the eyelids are still fused shut. The circulatory system and urinary tract are fully functioning.
By the end of week 16, most women have a visible bump. Your healthcare provider can feel the top of the uterus about 4 centimetres below your navel. Maternity clothes are no longer optional for most.
The "Quickening" Window
Somewhere between weeks 16 and 22, you will feel your baby move for the first time. First-time mothers often describe it as fluttering, bubbling, or a light tapping. Women who have been pregnant before tend to recognize it earlier, sometimes as early as week 14 or 15. An anterior placenta (positioned at the front of the uterus) can delay the sensation because it cushions the baby's movements. If you have not felt anything by week 22, mention it to your provider.
8 min read
Skin Changes and Belly Care
As your belly grows during the 4th month, your skin is adapting in visible ways. A comprehensive review published in Cureus (2024) catalogued the most common pregnancy-related skin changes. Here is what you might notice this month.
The Linea Nigra
A dark line running vertically from your navel to your pubic bone (and sometimes above the navel too). It is caused by increased melanin production from pregnancy hormones. Nearly every pregnant woman develops some degree of this line, though it is more visible on darker skin tones. It fades within a few months after delivery.
For trimester-specific sleep guidance, see the best sleeping position during pregnancy and left-side sleeping in pregnancy.
Darkening of Existing Pigmentation
Moles, freckles, areolae, and the skin around your genitals may darken. This is the same melanin surge responsible for the linea nigra. Most of it reverses postpartum, though some changes may be permanent.
Skin Itching
As skin stretches across your abdomen, it can feel tight and itchy. Moisturizing helps. Fragrance-free lotions or natural oils like coconut or almond oil applied after showering keep skin hydrated and reduce itching. If itching becomes severe, especially on your palms and soles, tell your doctor, as this can occasionally indicate a liver condition called intrahepatic cholestasis of pregnancy.
Pregnancy "Glow"
The increased blood volume (up 40 to 50% by now) gives skin a flushed, dewy appearance. Combined with oil production from hormonal changes, this creates what people call the pregnancy glow. Not everyone experiences it, and some women get acne instead. Both are normal.
Stretch Marks: What Actually Works
Stretch marks are one of the biggest concerns for pregnant women, and the advice around them is full of marketing claims that outpace the evidence.
Stretch marks (striae gravidarum) are tears in the dermis, the middle layer of skin, caused by rapid stretching. They appear as reddish-purple lines, usually on the abdomen, breasts, hips, and thighs. The Mayo Clinic notes that they affect up to 90% of pregnant women.
Here is what the research actually says:
| Approach | Evidence | Verdict |
|---|---|---|
| Keeping skin moisturized | May reduce itching and improve skin elasticity | Helpful for comfort, unproven for prevention |
| Cocoa butter creams | Multiple studies show no benefit over plain moisturizer | Feel good but do not prevent marks |
| Vitamin E oil | Limited evidence, may reduce severity slightly | Worth trying, unlikely to prevent entirely |
| Hyaluronic acid creams | Some evidence of modest benefit in early use | More promising than cocoa butter |
| Gradual weight gain | Rapid weight gain increases stretch mark risk | The most evidence-supported approach |
| Genetics | Strongest predictor of stretch marks | If your mother had them, you likely will too |
The honest truth is that genetics play the largest role. If your mother had significant stretch marks during pregnancy, your chances increase. Gradual, steady weight gain within your healthcare provider's recommended range is the most evidence-supported way to reduce their severity.
Brad, Owner, 40+ years of experience: "I mention this to expecting parents not because it is mattress-related, but because we have had this conversation in the showroom more times than you would think. Someone will be testing a mattress and mention they cannot get comfortable because their belly itches. A good moisturizing routine and breathable bedding, those are the practical things that help with the day-to-day discomfort."
Physical Activity and Your Growing Bump
The 4th month is often when exercise feels good again after the fatigue of the first trimester. The Society of Obstetricians and Gynaecologists of Canada recommends 150 minutes of moderate-intensity exercise per week throughout pregnancy, unless your provider advises otherwise.
Good options at this stage:
- Walking. Simple, free, and effective. A 30-minute walk improves circulation, mood, and sleep quality.
- Swimming. Water supports your belly weight, reduces joint strain, and helps with swelling. The buoyancy feels amazing as the bump grows.
- Prenatal yoga. Focuses on pelvic floor strength, hip flexibility, and relaxation breathing. Particularly helpful for improving sleep.
- Stationary cycling. Low impact, controlled, and easy to adjust intensity.
Avoid contact sports, activities with fall risk (skiing, horseback riding), and exercises that involve lying flat on your back for extended periods after about 16 weeks. If you were active before pregnancy, you can generally continue modified versions of your routine. If you are starting fresh, begin gradually.
A meta-analysis published in Sleep Medicine Reviews (2019) found that regular exercise during pregnancy was associated with improved sleep quality. Even modest amounts help.
Preparing for Better Sleep
Sleep in the second trimester is generally better than the first or third, but "better" does not mean effortless. Research published in the International Journal of Environmental Research and Public Health found that women averaged about 8 hours of sleep during the second trimester, but quality varied widely.
The 4th month is your preparation window. What you set up now pays off for the next 5 months.
Your Mattress
If you have been meaning to replace an old mattress, do it now while you can still comfortably test options. Side sleeping is becoming the recommended position, and a mattress that creates pressure points at the hip will only get worse as you gain weight. We see this pattern constantly at Mattress Miracle. Parents come in at 7 or 8 months barely able to lie down to test. At 4 months, you can take your time, try different firmness levels, and make a confident choice.
Research in Sleep Health (Bjorvatn et al., 2018) confirms that bedroom habits and environment are central to managing pregnancy-related sleep disruption, so building the right setup early pays off through the third trimester.
Our Restonic ComfortCare at $1,619 for the queen is popular with pregnant customers because the 1,222 individually wrapped coils adapt to changing weight distribution as pregnancy progresses.
Your Pillow Setup
Start with a regular pillow between your knees for hip alignment. As the bump grows over the coming weeks, you may want to add a thin pillow under the belly. Full-body pregnancy pillows become most useful from about week 20 onward.
Your Bedroom Environment
- Temperature: 18 to 20 degrees Celsius (you will run warm)
- Sheets: breathable cotton or bamboo blends
- A mattress protector (night sweats, water breaking later, and postpartum recovery all benefit from protection)
- A dim nightlight for inevitable bathroom trips
- Water and crackers on the nightstand for midnight nausea or hunger
Making the Most of the Second Trimester
If you are in the Brantford area and feeling good this month, take advantage of the energy. Book your 20-week anatomy scan at the local imaging clinic if you have not already. Check out prenatal fitness classes at the Wayne Gretzky Sports Centre. And if you need a new mattress, stop by our showroom at 441 1/2 West Street. Talia can help you test options while you are still comfortable enough to spend time lying down and comparing.
Shop This Topic at Mattress Miracle
Popular picks at Mattress Miracle:
- Restonic ComfortCare Dalton & Albany pocket-coil mattress
- Whitney bamboo mattress
- Somnia posture pillow
Or compare our mattresses in our Brantford showroom.
Visit Our Brantford Showroom
We are located at 441½ West Street in downtown Brantford. Free parking available. Our team does not work on commission, so you get honest advice based on your needs.
Mattress Miracle , 441½ West Street, Brantford, ON · (519) 770-0001
Hours: Monday–Wednesday 10am–6pm, Thursday–Friday 10am–7pm, Saturday 10am–5pm, Sunday 12pm–4pm.