Babies cry in the womb, and 18 other surprising facts I learned when I became a dad

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.

However, 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.

Source: US National Library of Medicine/MedlinePlus, Oakland University

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 2 weeks after a period, on average, and fertilization happens within 24 hours of that. This means if you're 8 weeks pregnant, your baby is about 15 days younger than that.

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.

Sources: American Pregnancy Association, "Williams Obstetrics", Business Insider

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 5 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 8 months and 24 days, not 9 months.

Source: Business Insider

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.

Sources: University of California San Francisco Medical Center, NYU School of Medicine

By week 8, a baby's heart beats regularly around 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 around week 8 or 9), I laughed out of shock.

Source: Mayo Clinic

Most of the ear structures required to pick up sound are formed by week 16. From then on, a mom's heartbeat, eating, breathing, talking, walking, exercising, and even digestion 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.

Sources: Pediatrics, CDC, WhatToExpect.com

The sounds a mom exposes herself to are generally what a baby is exposed to as well, but babies can't put in ear plugs.

The CDC says moms should, if at all possible, avoid very loud noises exceeding 115 dBA — like chainsaws, gunfire, jet engines, very loud concerts, and so on.

Consistent loud noise (like heavy machinery) can also damage a baby's hearing in the womb.

Sources: Pediatrics, CDC, WhatToExpect.com

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 week 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.

Sources: Office on Women's Health, WhatToExpect.com

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 start to pee inside the amniotic sac around week 8, 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.

Cheers, kids.

Sources: March of Dimes, Nature, American College of Obstetricians and Gynecologists, "Gray's Anatomy"

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.

Sources: "The Informed Parent: A Science-Based Resource for Your Child's First Four Years", "Origins: How the Nine Months Before Birth Shape the Rest of Our Lives"

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.

Sources: "The Informed Parent: A Science-Based Resource for Your Child's First Four Years", American Journal of Clinical Nutrition, Digestion

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.

Sources: Columbia University, HealthyChildren.org, American Pregnancy Association, Clinics in Perinatology

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.

Source: "Origins: How the Nine Months Before Birth Shape the Rest of Our Lives"

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.

Source: American College of Obstetricians and Gynecologists, March of Dimes, Materia Socio Medica

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.

Sources: Mayo Clinic, Rice University, "Origins: How the Nine Months Before Birth Shape the Rest of Our Lives"

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 16-23%.

Sources: March of Dimes, "Origins: How the Nine Months Before Birth Shape the Rest of Our Lives"

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.

Source: "Origins: How the Nine Months Before Birth Shape the Rest of Our Lives"

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.

Sources: ADC Fetal & Neonatal, Discovery Channel (via YouTube)