How Pregnancy Starts: From Sperm + Egg to Positive Test
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Sep 2, 2025
Overview
Human reproduction is incredibly detailed and precisely timed. From the moment sperm and egg meet, everything must go right from fertilization and cell division to implantation in the uterus and the formation of the placenta. Hormones, genes, and the body’s natural screening systems all work together to protect the chances of a healthy pregnancy. But because the process is so delicate, it’s also very common for early losses to happen naturally, even before anyone knows they were pregnant.
Step 1: Fertilization – When Sperm Meets Egg
1. Where Fertilization Happens
Fertilization usually happens in the ampulla, which is the wide part of the fallopian tube. That’s where sperm (after being released during sex) meets the egg that was just released from the ovary. But before sperm can fertilize an egg, it must go through a process called capacitation, which helps the sperm become able to break through the egg’s protective layers (like the sperm’s "training session" inside the female body).
(Georgadaki et al., 2016; Evans & Florman, 2002)
2. How the Sperm Gets In
The sperm uses special enzymes and strong movement to push through the cumulus oophorus (a layer of cells around the egg) and the zona pellucida (a tough outer shell). Then it reaches the egg’s surface (Yanagimachi, 2022; Trebichalská & Holubcová, 2020).
3. The “Handshake” That Allows Entry
There’s a kind of molecular “handshake” that has to happen: a protein called IZUMO1 on the sperm connects with a receptor called JUNO on the egg. If this connection doesn’t happen, fertilization fails (Aydin et al., 2016; Ohto et al., 2016; Bianchi et al., 2022).
4. Zygote Formation: The First Cell of New Life
Once the sperm enters, the egg finishes its last cell division, and the genetic material from both parents combines. This creates a new single cell called a zygote, which has a complete set of DNA (Ezoe et al., 2022; Coticchio et al., 2022).
5. Blocking Extra Sperm
The egg quickly blocks other sperm from entering – this is called the polyspermy block. Without it, the embryo wouldn’t develop correctly (Bianchi & Wright, 2016; Ohto et al., 2016).
Step 2: Establishing a Pregnancy
Once fertilization is complete and a zygote (the first cell of a new life) is formed, the body begins the next phase: establishing a pregnancy. This stage involves a series of transformations as the zygote travels toward the uterus, embeds itself in the uterine lining, and starts forming the placenta.
Stage 1: Early Cell Divisions and Blastocyst Formation
After fertilization, the zygote travels down the fallopian tube toward the uterus. Along the way, it divides into more and more cells through a process called mitosis.
By the time it reaches the uterus (around 5–6 days after fertilization), it has become a blastocyst (a tiny hollow ball of cells). Inside the blastocyst:
The inner cell mass will become the embryo
The outer layer (called the trophectoderm) will become the placenta
Stage 2: Implantation into the Uterine Lining
Next, the blastocyst needs to implant into the lining of the uterus. This happens during a short period known as the “implantation window”, when the uterus is most receptive.
The blastocyst first attaches to the uterine wall, then begins to invade the tissue so it can establish a connection and start receiving nutrients (McEvoy & Tetrokalashvili, 2019).
Stage 3: Decidualization – Preparing the Uterus to Support the Embryo
At the same time as implantation, a process called decidualization happens. This is when the cells of the uterine lining change into a special form that:
Feeds and supports the embryo
Helps regulate the immune system so it doesn’t attack the embryo (which has foreign DNA from the father)
Even helps “screen” embryos and reject poor-quality ones (Okada et al., 2018; Ng et al., 2020)
Stage 4: Placentation & Building the Lifeline
As the embryo becomes more developed, the placenta starts to form from the outer layer of the blastocyst.
The placenta is a temporary organ that transfers nutrients and oxygen from the mother to the embryo, removes waste and sends out hormones that help maintain the pregnancy and support fetal development (Gauster et al., 2022; Deng et al., 2021).
How Pregnancy Is Detected
Once the embryo successfully implants in the uterus, the body begins producing a hormone called human chorionic gonadotropin (hCG). This hormone is only made during pregnancy and is what most pregnancy tests detect.
Home urine tests can usually detect hCG about 10–14 days after ovulation—this is typically around the time of a missed period.
Blood tests, which are done in a clinic, can detect smaller amounts of hCG and may give a reliable result a few days earlier than urine tests.
A positive test means implantation has likely occurred and the body has begun supporting a pregnancy. However, in very early stages, false negatives are possible if testing is done too soon, and early losses can still occur.
Why Not Every Fertilized Egg Becomes a Baby: Genetics and Natural Selection
Even if a sperm successfully fertilizes an egg, it doesn’t guarantee a pregnancy. In fact, many fertilized eggs are lost very early on, often before the person even knows they were pregnant. This usually happens because something is wrong with the embryo’s chromosomes (the parts of cells that carry DNA) or because the uterus isn’t ready to support the embryo at that moment (Zinaman et al., 1996; Teklenburg et al., 2010).
The lining of the uterus can actually act like a biological filter. It checks whether the embryo is developing normally. If the signals from the embryo are weak or irregular, the uterus may reject it before it can implant. This is the body’s way of doing quality control—rejecting embryos that likely wouldn’t grow into healthy babies (Brosens et al., 2022).
In some cases, the problem starts even earlier, during fertilization itself. Some people have genetic mutations that stop the sperm and egg from joining properly. For example, a mutation in a gene called WEE2 can prevent them from fusing at all (Sang et al., 2018). Also, right after fertilization, the DNA from the sperm and egg must combine at exactly the right time. If this process is off, the embryo won’t survive (Coticchio et al., 2022).
Today, doctors can use genetic testing in fertility clinics to check embryos before implantation. This helps choose embryos with the best chances of developing successfully, especially during in vitro fertilization (IVF) treatments.
