- Ann Price Photography
Happy father’s day to all the dads and dads-to-be out there. When I learned I was going to be a new dad, I was floored … and scared.
To counter the fear of becoming new parents, my wife and I read. And read and read and read.
We rounded up about a dozen science-backed books, scanned countless research studies, downloaded pregnancy apps, and shared hundreds of articles with each other over the following months.
During that flood of information, a handful of surprising facts floated above the rest, and I’ve collected them here.
What follows may not surprise an obstetrician, or even seasoned parents, but it nonetheless highlights the extraordinary human journey that is pregnancy.
Fertilization is 1-in-100-million affair.
- REUTERS/Anthony Bolante
Men release about 100 million sperm each time they ejaculate, though the semen of some especially fertile men can contain hundreds of millions of sperm.
But only a few hundred may ever reach a woman’s egg. Special receptors on the surface of an egg make sure only one gets in. (Fraternal twins occur when two sperm fertilize two eggs, while identical twins occur when one sperm fertilizes one egg and it splits into two embryos.)
Babies are about 15 days younger than the length of a pregnancy.
- Alexandre Meneghini
Since 1836, doctors have marked the first day of a woman’s last menstrual period as the first day of pregnancy, or “gestational age,” not when a sperm fertilizes an egg.
Ovulation happens about two weeks after a period, on average, and fertilization happens within 24 hours of that. This means if you’re eight weeks pregnant, your baby is about six weeks old.
Doctors still use gestational age, not ovulation age (also called postconceptional age) because it’s hard to detect ovulation and fertilization even more so. Periods, meanwhile, are hard to miss – and easier to notice when they’ve gone missing.
Most women aren’t pregnant for 9 months.
- Getty Images
Nine months works as a very rough estimate, but this oft-said number can lead to a number of misconceptions.
First, it’s not a target; a healthy pregnancy can vary as much as five weeks around a 40-week due date. In fact, only 4% of women deliver on their 40-week due date.
The typical pregnancy – measured from ovulation, not the last menstrual period (which is standard) – actually lasts about eight months and 24 days, not nine months.
Source: Business Insider
Babies float around in the womb for the first week of life.
It takes the first tiny ball of dividing cells a few days to move down the Fallopian tubes and reach the uterus, and another few days for the embryo to implant itself.
From there, it embeds into the cushy wall of a woman’s uterus, soaks up nutrients, and triggers a cascade of further development.
But up until that point, babies are womb drifters.
A developing baby’s heart starts pumping blood at six weeks.
- Getty Images
By week eight, a baby’s heart beats regularly about 160 times a minute. The pumping is also audible with the help of an ultrasound device.
When I first heard my baby’s heartbeat through ultrasound, at about week eight or nine, I laughed out of shock.
Source: Mayo Clinic
Babies can hear inside (and outside) the womb, and the uterus is very noisy.
- Thomson Reuters
Most of the ear structures required to pick up sound are formed by week 16. From then on, a mom’s heartbeat, eating, breathing, walking, talking, exercising, burping, and digestive gurgling can easily be heard by a developing baby.
This may help explain why babies find noise so comforting. There’s also some evidence to suggest babies learn to recognize and react to mom’s voice while inside the womb.
Loud noises can damage a fetus’ hearing.
- REUTERS/David McNew
The sounds a mom exposes herself to are what a baby is exposed to as well, but babies can’t put in ear plugs.
The CDC says moms should avoid very loud noises exceeding 115 dBA – chainsaws, gunfire, jet engines, blaring music, loud concerts, and so forth.
Consistent loud noise (like heavy machinery) can also damage a baby’s hearing in the womb.
Babies open their eyes inside the womb and can see light from the outside.
Although a baby’s eyes can “see” light starting around week 16, their peepers aren’t fully formed until about week 20. The eyes first open between weeks 26 and 28.
Their vision is rather blurry, but they can see – and respond with a flutter of activity to – bright sources of light like the sun or a flashlight pointed at a woman’s belly.
Getting outside often might even help a baby’s eyes develop and reduce the risk of a few eye disorders.
Women may become more easily disgusted, and possibly fearful of outsiders, during pregnancy.
- Giuseppe Milo/flickr
Morning sickness is no fun, but a popular scientific explanation for all those waves of disgust, nausea, and vomiting suggests that it helps protect a woman’s fetus during her first trimester (or first 12 weeks).
This “disease-threat” model goes something like this: Anything that looks or smells or feels or tastes funny is more likely to gross out a pregnant woman, thus protecting her developing baby from toxic and potentially infectious things while her immune system is partially weakened.
A study in 2007 took that idea a step further, asking if pregnant women are more likely to instinctively avoid foreigners, strangers, and other “outgroups.” It found some preliminary though inconclusive evidence in a group of 206 pregnant women, suggesting they may perceive “outgroups” more negatively during the first trimester.
Sources: “Origins: How the Nine Months Before Birth Shape the Rest of Our Lives“, Philosophical Transactions of the Royal Society B, The American Naturalist, American Pregnancy Association, Evolution and Human Behavior
Babies swim in and drink their own pee for about 25 weeks.
- Dave Mosher
Babies start to pee inside the amniotic sac around week eight, though urine production really picks up between weeks 13 and 16 (when kidney development is more complete).
They can start drinking this mix of pee and amniotic fluid around week 10 or 11, or when a layer of cells blocking their mouths – called the buccopharyngeal membrane – ruptures, allowing the baby to swallow. By week 20 most of the amniotic fluid is urine.
Babies can taste and smell the food mom eats in the womb.
Flavor molecules from food that a mom eats pass from her blood, through the placenta, and into a baby’s amniotic fluid.
And since taste buds start to develop around week 11 (right as the mouth opens up), a baby can taste whatever mom eats from then on – though a sense of taste is stronger around weeks 28 and 29, when taste buds are fully mature.
Babies also snort up amniotic fluid, and it’s thought they can smell food, too.
Mom’s diet affects a baby’s future food preferences.
Since flavor molecules travel so readily, many studies have shown food preferences and aversions start in the womb in this way; if mom avoids certain foods, baby is likely to reject those flavors outside of the womb.
The relationship continues after birth, too, since flavor molecules regularly show up in breast milk.
One study even showed moms who consistently ate a lot of anise during pregnancy eventually gave birth to babies who preferred anise-flavored milk.
Most babies hold their poop until birth.
- Getty Images
Called meconium, a baby’s first stool is made of all the skin, hair, bile, proteins, white blood cells, and other stuff that floats in the amniotic fluid – because a baby drinks it all for 20 to 25 weeks.
Meconium starts forming as soon as a baby opens its mouth and begins swallowing amniotic fluid, around week 11, but meconium production really picks up by week 19 or 20 as a fetus matures.
It typically comes out after birth as a greenish-black, tarry, and odorless mess. Or at least this was the case for my baby; other colors (like white) can indicate a serious medical condition.
And while 13% of babies do poop in the womb, too much meconium in the amniotic fluid can block a baby’s airways before birth, leading to an oxygen-deprived state called fetal distress.
One cell layer is all that separates mom from baby.
The layer between a baby’s placenta and mom’s uterine wall is so thin to allow nutrients to easily pass from mom’s blood into a baby’s blood without ever touching.
Some moms can become allergic to future pregnancies.
- mahalie / Flickr, CC
The risk has to do with blood, and it starts with a genetically inherited factor called Rhesus, or Rh factor. When this protein shows up on the surface of blood cells, that person is Rh-positive. Meanwhile, Rh-negative people don’t have it.
If a dad is Rh-positive and a mom is Rh-negative, future pregnancies can be tougher to keep – if mom’s Rh-negative blood ever mixes with her Rh-positive baby’s blood.
While usually not a problem during the first pregnancy, mom’s and baby’s blood can mix during the disarray of birth, causing mom to develop “Rh sensitization” – when she makes antibodies that may attack future Rh-positive babies, causing miscarriages.
About 2.3% of pregnancies are at risk of this Rh sensitization, but only 1% of those moms at risk actually develop the antibodies.
Luckily, a shot of Rh immunoglobulin between week 28 and up to 72 hours after a mom’s first birth can prevent the problem altogether.
A woman’s blood volume can increase by 30% to 50% over a pregnancy.
- Thomson Reuters
This is why feet swell and pains come out of seemingly nowhere for many pregnant women. To make room for all the new fluid – which helps consistently nourish a developing baby, and carry away its waste – a hormone called relaxin softens and expands mom’s blood vessels and heart.
These softened vessels can also lead to hemorrhoids and varicose veins.
Traumatic emotional events for mom can deeply affect developing babies.
- Machiel van Zanten/Flickr
Episodes of traumatic stress experienced by a pregnant mom can have surprising effects on her baby. Kids born to parents with post-traumatic stress disorder (PTSD), for example, are more likely to develop PTSD in their own lifetimes – despite not being exposed to more traumatic events than others.
Mom losing a family member during a pregnancy also raises the likelihood of a premature delivery by as much as 23%.
A mechanism for this link remains elusive.
The more social support a mom has, the heavier (and healthier) her baby tends to be.
- Getty Images
The correlation between social support and higher birth weight isn’t perfectly understood, but moms with a reliable squad may be less stressed and more likely to sleep better, eat better, and make other lifestyle choices that improve birth weight.
Having less of a certain stress hormone called cortisol around might also benefit the baby.
Babies cry in the womb.
Seeing is believing for this one.
Researchers accidentally made the discovery while studying mothers who used cigarettes or cocaine. After playing a sound on the pregnant mothers’ bellies, ultrasound videos showed the babies startling, opening their mouths, and gasping.
Even the dreaded (and cute) quiver chin is visible.