4 Month Pregnancy Belly: Size, Shape and Comfort

Reading Time: 10 minutes

Four months. You probably spent the first trimester wondering when the bump would show up, and now here it is. Pants that fit two weeks ago do not button anymore. Strangers are starting to ask. And your body, which spent the last few months looking mostly the same while doing extraordinary internal work, is finally showing it.

Month four marks the beginning of the second trimester, often called the "honeymoon period" of pregnancy. Morning sickness usually fades, energy returns, and for many women, sleep improves compared to the exhausting first trimester. But your growing belly introduces new considerations, from how you sit at work to how you position yourself at night.

Your Belly at Four Months: What Has Changed

The biggest visible change between months 3 and 4 is that your uterus has risen out of the pelvis. Before this, the uterus was tucked behind the pubic bone. Now it has climbed above the pelvic brim and is pushing forward against your abdominal wall. Your healthcare provider can feel the top of the uterus (the fundus) about 4 centimetres below your navel at 16 weeks.

For most first-time mothers, this is when the bump becomes unmistakable. Second-time mothers likely started showing a few weeks earlier, as their abdominal muscles had already been stretched by a previous pregnancy.

Weight gain by this point is typically 2 to 4.5 kilograms total, depending on your pre-pregnancy weight and your healthcare provider's recommendations. Health Canada suggests a total pregnancy weight gain of 11.5 to 16 kilograms for women who started at a healthy BMI, but individual guidance from your provider is more useful than general ranges.

Baby Development During Month Four

Month four brings dramatic developmental milestones. Your baby has graduated from embryo to fetus a while ago, and the organs formed in the first trimester are now maturing and beginning to function.

Skeleton. Bones are hardening from cartilage. The skeleton becomes visible on ultrasound. The baby can make sucking motions and may start swallowing amniotic fluid.

Movement. The baby is moving actively, stretching, kicking, and even somersaulting. You may begin to feel the first fluttering movements, called quickening, toward the end of month four. First-time mothers often describe it as a bubbling or fluttering sensation that can easily be mistaken for gas.

Skin and hair. A fine layer of hair called lanugo covers the skin. This keeps the baby warm and will mostly shed before birth. The skin is thin and translucent, with blood vessels visible underneath.

Facial features. Eyes have moved to the front of the face. Eyebrows and eyelashes are forming. The ears are nearly in their final position and the baby may begin responding to external sounds.

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The Second Trimester Shift

The transition from first to second trimester brings welcome changes for most women. Hormones stabilize, nausea fades, and the crushing fatigue of the first 12 weeks often lifts. But new symptoms take their place as your body adapts to a growing belly.

What Typically Improves

• Morning sickness usually eases or disappears by week 14 to 16
• Energy levels rebound as the placenta takes over hormone production
• Breast tenderness often decreases
• Mood stabilizes as hormonal fluctuations settle
• Appetite returns, sometimes enthusiastically

What May Start

• Round ligament pain (sharp pulling sensations on the sides of the lower abdomen)
• Nasal congestion from increased blood volume (pregnancy rhinitis)
• Visible veins on breasts and abdomen
• The linea nigra, a dark vertical line from navel to pubic bone
• Back pain as your centre of gravity shifts forward
• Growing difficulty finding a comfortable sleep position

Sleep in the Second Trimester

Most research agrees that the second trimester is generally the best period for sleep during pregnancy. A longitudinal study published in the International Journal of Environmental Research and Public Health (2022) found that total sleep time averaged about 8 hours during the second trimester, compared to more disrupted patterns in the first and third trimesters.

But "best" is relative. Research in the Journal of Obstetric, Gynecologic & Neonatal Nursing found that nearly 28% of pregnant women still slept fewer than 7 hours per night during the second trimester. And a separate study linked sleeping 7 hours or fewer in the second trimester to increased preterm birth risk.

Sleep Position at 4 Months

This is when the sleep position conversation gets real. The Society of Obstetricians and Gynaecologists of Canada (SOGC) recommends that pregnant women begin sleeping on their side, particularly the left side, from mid-pregnancy onward. Left-side sleeping optimizes blood flow to the uterus and placenta by keeping the weight of the uterus off the inferior vena cava, the large vein returning blood from the lower body to the heart.

At 4 months, back sleeping is generally still considered safe for most women, but this is a good time to start practising side sleeping. If you have always slept on your back or stomach, the transition can be uncomfortable. Pillow placement makes the biggest difference.

Mattress Considerations

Your body weight distribution is shifting. At 4 months, the extra weight is concentrated around your midsection, which changes how your mattress supports you. Side sleepers, in particular, need a mattress that cushions the hip and shoulder without letting the spine sag.

If you are waking with hip pain or lower back stiffness, your mattress may not be providing enough pressure relief. At Mattress Miracle, we carry the Restonic ComfortCare (queen, $1,125, 1,222 coils) which uses zoned support to provide firmer support in the lumbar region and softer cushioning at the hips and shoulders. For couples where one partner is pregnant and the other is not, individually wrapped coils prevent motion transfer so one person's tossing and turning does not wake the other.

Staying Comfortable as Your Bump Grows

Clothing

Month four is typically when regular clothes stop working. Maternity pants with stretchy waistbands, longer tops that accommodate the bump, and supportive undergarments become daily essentials. A good maternity bra with wide straps and no underwire is worth investing in now, as your ribcage and breast size will continue changing.

Exercise

The Society of Obstetricians and Gynaecologists of Canada recommends at least 150 minutes of moderate-intensity physical activity per week during pregnancy, spread across at least three days. Walking, swimming, prenatal yoga, and stationary cycling are all safe options. Regular exercise during pregnancy is associated with better sleep quality, according to a meta-analysis published in Sleep Medicine Reviews (2019).

Back Pain Management

As your belly grows, your centre of gravity shifts forward. Your lower back compensates by curving inward more than usual (increased lumbar lordosis). This is a common cause of second trimester back pain. Helpful strategies include:

• Prenatal stretching (cat-cow, pelvic tilts) before bed
• Avoiding standing in one position for long periods
• Wearing supportive, low-heeled shoes
• Sitting with lumbar support (a small cushion or rolled towel works)
• Sleeping on a mattress that maintains spinal alignment

Heartburn and Sleep

Heartburn often starts in the second trimester as the growing uterus pushes the stomach upward and progesterone relaxes the sphincter between the esophagus and stomach. Eating smaller meals, avoiding spicy or acidic foods near bedtime, and elevating the head of your bed can help. An adjustable bed base is particularly useful here, letting you raise your upper body without stacking pillows that slip during the night.