The Truth About Tracking Your Baby Size by Week in Pregnancy

It was a Tuesday at like 9:15 AM, and I was exactly 24 weeks pregnant with Leo. I was sitting in the terrifyingly quiet waiting room of my OB-GYN’s office, sweating straight through my favorite grey Zara sweater even though it was freezing in there. My pregnancy app had just cheerfully pinged to inform me that my unborn child was currently the size of a rutabaga. I was holding a lukewarm iced coffee from Dunkin, watching the condensation drip onto my maternity jeans, and frantically Googling "what the hell does a rutabaga weigh" with my free hand. My husband, Dave, was sitting next to me completely unbothered, reading a vintage car magazine from 2014 like we weren't about to get an ultrasound that would allegedly tell us if our kid was growing normally.

The biggest myth we're sold during pregnancy is that tracking how big your infant gets each week is this exact, adorable science paved with perfectly scaled grocery store produce. It’s crap. Total crap. The apps tell you your kid is a sweet potato one day and a cantaloupe the next, and somehow by month eight they're a bunch of leeks? How do you even measure a baby against a leek? Is it a volume thing? A length thing? Anyway, the point is, I spent half my first pregnancy absolutely terrified because the internet told me Leo should be the size of an eggplant, but my stomach still just looked like I had eaten a really aggressive burrito for lunch.

You download these trackers thinking they'll give you control, but honestly, they just gave me a complex about farmers' markets.

What the hell is a rutabaga anyway

Let's just talk about the fruit and vegetable comparisons for a second because I need to rant. During my first trimester, the app said my baby was the size of a poppy seed. Okay, cute. I can visualize that. But then by week 15, we were suddenly at an apple, and by week 20, a banana. Which, logistically, makes zero sense. Is it a long banana? A thick banana? The sheer panic I felt when Maya’s pregnancy app jumped from "large mango" to "head of cauliflower" in what felt like three days was absurd.

I'd literally stand in the produce aisle at Whole Foods holding a squash against my stomach, trying to figure out if my uterus was expanding correctly, while strangers judged me. Meanwhile, at my 30-week appointment, my doctor just slapped a standard tape measure on my belly—like the kind you use for sewing—and was like "looks good," which seemed scientifically dubious but whatever.

How Dr. Miller actually figured out the weight thing

So when we finally went back for the actual anatomy scan, the ultrasound tech was aggressively jabbing my stomach with the wand, and I asked her how they *actually* know the size. Because clearly, the rutabaga metric wasn't cutting it.

My pediatrician later explained this to me too, but basically, in the beginning, they measure them from the top of their head to their little butt. It's called crown-to-rump length, I think? Which makes sense because for the first few months, they're basically curled up like tiny shrimp in there. But then after the first trimester, they stretch out, and they start measuring the head width and the femur bone. Biparietal diameter is the head thing. It sounds like a dinosaur. I remember sitting there while Dr. Miller squinted at the blurry grey screen, clicking a little mouse to measure a white line that was supposedly Leo's thigh bone, and my entire heart was in my throat hoping the numbers matched up with whatever week I was supposedly on.

And the wild part is, they just dump all these random bone measurements into an algorithm, and the computer spits out an estimated weight. But it's an *estimate*. A guess. Dr. Miller told me it could be off by like a pound or more in either direction. So basically, science is just guessing, wrapping it in a fancy medical term, and charging us for the privilege.

Dave’s genetics ruined my birth plan

I spent so much time worrying that I wasn't eating enough of the right things, or that my morning coffee habit (which I refused to give up, don't judge me) was stunting the baby's growth. I was constantly analyzing every little thing.

But around week 32 with Leo, he started measuring massive. Like, off-the-charts big. I freaked out. I thought I had gestational diabetes or that my placenta was malfunctioning. I spent three days crying into my decaf latte. But my doctor just looked at Dave—who's 6'2, has shoulders like a linebacker, and basically looks like a lumberjack—and then looked at me, and said, "Genetics. You aren't going to have a tiny fairy baby." Which, rude, but accurate.

It turns out, your own height and the baby's dad's height play a massive role in how they grow inside you. Plus, Leo was a boy, and apparently, they tend to be a little heavier. So all that anxiety for nothing, just because the app said he should be the size of a pineapple and he was already measuring like a Thanksgiving turkey.

The absolute chaos of buying clothes for an imaginary size

Here's where the real problem with not knowing their exact size hits you: trying to buy baby clothes. When I was pregnant with Maya, I was determined to be "ready." I had washed and folded tiny little socks. I had organized drawers by month. But if an ultrasound is essentially a sophisticated guessing game with a plus-or-minus margin of an entire bowling ball, buying clothes is a total shot in the dark.

With Leo, I bought all these stiff, structured newborn outfits that looked adorable on the hanger but were a nightmare in reality. When we finally brought him home, he was huge, and those outfits left terrible red marks on his sensitive skin. He screamed every time I tried to jam his arms into non-stretchy synthetic sleeves.

By the time I had Maya, I threw all that out. I realized you need things that forgive you if you get the sizing wrong. I ended up getting a few of the Organic Cotton Baby Bodysuit Sleeveless Infant Onesies from Kianao. BEST decision. Honestly. Because they're 95% organic cotton and 5% elastane, they've this magical stretch to them. Maya was born smaller than Leo, but these onesies didn't hang off her weirdly, and as she chunked up over the first few months, the fabric just stretched with her. Plus, there are no scratchy tags, and you don't have to wrestle them over a screaming newborn's head because they've those little envelope shoulders. Seriously, if you're staring at a pile of baby clothes right now panicking about sizes, just get something soft that stretches.

I also bought the Flutter Sleeve Organic Cotton Baby Bodysuit Ruffled Infant Romper for her because I'm a sucker for anything with a ruffle. The organic cotton is super soft and breathable, which is great because Maya ran hot like a little furnace. But honestly? The flutter sleeves are really cute for pictures, but when she was doing that frantic newborn squirming thing, they sort of rode up to her ears. It's adorable, but maybe more for when they're a little older and not constantly flailing like an overturned turtle.

The things that actually matter when they get here

Eventually, they do come out, and you stop caring about what vegetable they resemble. You realize that the only things that matter are surviving the night and keeping them relatively comfortable.

If you're nesting right now and aggressively adding things to your cart at 2 AM while worrying about ultrasound percentiles, you can browse some really useful organic baby essentials over at Kianao. It's way better for your mental health than Googling fetal femur lengths.

When Maya started teething around six months, her growth curve slowed down a bit because she was so miserable she didn't want to eat. I panicked, obviously. I bought the Squirrel Teether Silicone Baby Gum Soother from Kianao. It has this little acorn design that I thought was hilarious. Look, it's a fine teether. It's made of food-grade silicone, no weird chemicals, which gave me peace of mind when she shoved the entire thing into her mouth. But I'll be honest, it didn't magically stop the 3 AM screaming fits. Maya would chew on the squirrel's tail for maybe five minutes of blessed silence, and then violently throw it at our cat. It’s okay, but it’s not a miracle worker. Nothing is.

What *was* a miracle was finding a blanket that didn't make me worry she was suffocating or overheating. We had the Organic Cotton Baby Blanket with Polar Bear Print, and I swear by this thing. It's double-layered but so breathable. During those late-night feeds when I was functioning on zero sleep and just staring at the wall, having her wrapped up in this soft, chemical-free cotton made me feel like I was at least doing one thing right. And the polar bears are cute without being aggressively obnoxious like some baby prints.

So instead of panicking over app notifications and buying twelve rigid newborn outfits and measuring your stomach in the bathroom mirror trying to guess if they grew an inch, maybe just drink some water, buy a couple of stretchy onesies, and accept that they're going to be whatever size they're going to be. If you're staring at an ultrasound photo right now and wondering how this little alien is going to fit into the real world, take a breath, close the fruit app, and check out Kianao's newborn collection to get some forgiving, stretchy essentials so you're covered either way.

Why is my kid measuring two weeks ahead at the ultrasound?

Oh god, I had this exact panic attack with Leo. My doctor literally laughed at me (gently, but still). She told me that those ultrasound dates are just averages based on a computer model. If your baby is measuring a week or two ahead, it usually just means they're genetically predisposed to be a bit bigger, or the tech clicked the mouse a millimeter too far on the screen. It doesn't mean you're going to birth a toddler. My pediatrician said unless the gap gets massive, a couple of weeks either way is totally normal.

Should I hoard newborn sizes or just buy 0-3 months?

Don't hoard newborn sizes! I learned this the hard way. Have like, maybe three newborn stretchy outfits just in case you deliver early or they're tiny. But Leo wore newborn clothes for exactly eight days before he hulked out of them. Mostly buy 0-3 months. Just make sure they're soft and stretchy so even if they're a little big at first, you can roll the sleeves. You'll blink and they'll fit.

Is the tape measure thing they do on your belly honestly science?

Fundal height! Yeah, it feels like they're measuring you for curtains, right? Honestly, my OB said it's just a super low-tech way to make sure the uterus is growing always between ultrasounds. It's not exact. One time my doctor got a smaller measurement just because the baby had dropped lower in my pelvis. It's just a screening tool, so don't freak out if it's off by a centimeter one week.

What if the app says they should be kicking but I don't feel anything yet?

The apps are the devil. With Maya, the app told me at 18 weeks I should be feeling "flutters." I felt nothing. I was convinced something was horribly wrong. It turned out I had an anterior placenta—which basically means the placenta was sitting right at the front of my stomach, acting like a giant pillow muffling all her kicks. I didn't really feel her until week 22. Your body isn't a textbook, so ignore the app's timeline.

Does eating sugar make the baby grow too fast?

Look, if you've diagnosed gestational diabetes, yes, that can definitely make the baby grow larger than average because of the blood sugar crossing the placenta. But if your glucose test was normal? My doctor told me that eating a brownie or having my daily coffee wasn't going to magically turn Leo into a giant. Your body is pretty good at regulating what the baby gets. Eat the pastry. You're growing a human, for hell's sake.