At five months of pregnancy (roughly 20 weeks gestation), a fetus is about 10 inches long from head to toe and weighs around 10 ounces, or just over half a pound. That’s about the size of a mango. This is also the point in pregnancy when many parents get their first detailed look at the baby during the mid-pregnancy ultrasound.
What “Five Months” Actually Means in Weeks
Pregnancy math is not intuitive. Doctors count pregnancy from the first day of your last menstrual period, which means you’re already considered “four weeks pregnant” by the time you miss a period. Because of this, five months of pregnancy doesn’t line up neatly with 20 calendar weeks. In practice, five months pregnant generally covers weeks 17 through 20 of gestation.
This also means the fetus is actually about two weeks younger than the gestational age suggests, since conception typically happens around two weeks into the menstrual cycle. A fetus at “20 weeks gestation” has really been developing for about 18 weeks.
Size Week by Week During Month Five
The fetus grows rapidly during this stretch. At 17 weeks, it’s roughly 7 to 8 inches long and weighs about 5 to 6 ounces. By 20 weeks, it has nearly doubled in weight, reaching around 10 ounces and stretching to about 10 inches measured from head to heel. To picture that length, think of a banana laid out straight.
Earlier in pregnancy, doctors measure a fetus from the top of the head to the bottom of the spine (called crown-to-rump length), because the legs are curled up tightly and hard to measure. By the middle of the second trimester, the limbs have stretched out enough that head-to-heel measurements become standard. That shift is one reason published fetus sizes can seem to jump dramatically around this stage.
What’s Developing at This Stage
Size is only part of the picture. At five months, a fetus is going through some of the most noticeable developmental changes of the entire pregnancy.
Around week 17, toenails begin forming. By week 18, the ears start standing out from the sides of the head, and the fetus may begin detecting sounds. The digestive system has started working for the first time. At week 19, the skin gets covered in a waxy, protective coating called vernix, which shields delicate skin from the amniotic fluid surrounding it. By week 20, the fetus has established a regular sleep-wake cycle and can be startled awake by loud noises or your movements.
This is also when many parents first feel the baby move, a sensation often described as fluttering or gentle tapping. First-time parents sometimes don’t recognize it until closer to week 22, while those who’ve been pregnant before may notice it as early as week 16.
The 20-Week Ultrasound
Most pregnant people have a detailed anatomy scan around week 20, and it’s one of the most thorough ultrasounds of the entire pregnancy. The technician takes four key measurements: the diameter of the head, the circumference of the head, the circumference of the abdomen, and the length of the thigh bone. Together, these measurements help confirm that the baby is growing on track for its gestational age.
Beyond size, the scan checks the brain, heart, spine, kidneys, and other organs for structural development. It’s also the appointment where you can typically learn the sex, if you want to know. The scan usually takes 30 to 45 minutes, longer if the baby is in an awkward position or if the technician needs additional views.
Why Size Can Vary
The numbers above are averages, and healthy fetuses come in a range of sizes. Genetics play the biggest role: taller parents tend to have longer babies, and birth weight often runs in families. Other factors include nutrition, how well the placenta is functioning, and whether you’re carrying multiples. A fetus measuring a week or two ahead or behind the average is common and rarely a concern on its own.
If measurements fall significantly outside the expected range, your provider may schedule follow-up ultrasounds to track growth over time. A single measurement that looks small or large matters less than the overall growth pattern across several weeks.