The most common cause of miscarriage at 4 weeks is a chromosomal abnormality in the embryo. Roughly 50% of all first-trimester losses are tied to genetic errors that prevent the embryo from developing normally. At this stage, the pregnancy is so early that it’s often called a chemical pregnancy: the embryo implanted just enough to trigger a positive test, but it stopped growing within days.
Why Most 4-Week Losses Are Genetic
When an egg and sperm combine, the resulting embryo needs exactly the right number of chromosomes to develop. Errors during that process are surprisingly common, and they’re the single biggest driver of early miscarriage. Among pregnancies lost to chromosomal problems, over 90% involve the wrong number of chromosomes rather than structural damage to the chromosomes themselves.
The most frequent error is autosomal trisomy, where the embryo ends up with three copies of a particular chromosome instead of two. This accounts for more than half of chromosomally abnormal miscarriages, with extra copies of chromosomes 16 and 22 being the most common culprits. Other patterns include monosomy X (where the embryo has only one sex chromosome instead of two, seen in about 11% of chromosomally abnormal losses) and triploidy, where the embryo carries an entire extra set of chromosomes.
These errors are random. They happen during cell division and typically have nothing to do with anything either parent did or didn’t do. The risk does increase with age, particularly after 35, but chromosomal miscarriages occur across all age groups.
What a Chemical Pregnancy Actually Is
A pregnancy loss at 4 weeks falls into a category called a chemical pregnancy or biochemical pregnancy. At this point, the embryo has implanted in the uterine lining and begun producing HCG, the hormone that turns a pregnancy test positive. But because it’s too early to see anything on ultrasound, HCG is the only evidence the pregnancy exists.
When the embryo stops developing, HCG levels drop instead of rising steadily. A test that was positive a few days ago may come back negative, or your period arrives roughly on time, possibly a bit heavier than usual. Many chemical pregnancies go unnoticed entirely. Before home pregnancy tests were sensitive enough to detect HCG this early, most people would have simply experienced what felt like a late or heavy period. The loss itself typically feels similar to a normal period: cramping and bleeding that resolves on its own within several days.
Low Progesterone and Implantation Problems
Progesterone plays a central role in the earliest days of pregnancy. After ovulation, the ovary produces progesterone to prepare and maintain the uterine lining so the embryo can implant and receive nutrients. If progesterone levels are too low during this window, the lining may not support the embryo well enough for it to survive.
Research consistently links low progesterone to first-trimester miscarriage. Multiple studies have found that women who miscarried had measurably lower progesterone levels than those whose pregnancies continued, and some researchers consider progesterone alone to be a strong predictor of early pregnancy loss. This connection is sometimes described as a luteal phase defect, meaning the second half of the menstrual cycle doesn’t produce enough hormonal support for implantation to succeed.
Beyond hormones, the uterine lining itself may not be receptive at the right time. Implantation can only happen during a narrow window, typically 6 to 10 days after ovulation. In some women, that window shifts or the lining expresses a different genetic profile that makes attachment less likely. Impaired receptivity of the uterine lining is estimated to account for roughly two-thirds of implantation failures.
Uterine and Structural Factors
Physical conditions in the uterus can also interfere with implantation or early embryo survival. Fibroids that push into the uterine cavity can prevent the embryo from attaching properly by increasing contractions, altering blood flow, and disrupting the chemical signals the lining uses to communicate with the embryo. Endometrial polyps are the most common uterine lesion linked to implantation failure, and they interfere not only by physically crowding the space but also by changing the chemical environment of the lining.
Scar tissue from previous uterine procedures can damage the functional layer of the lining. Congenital differences in uterine shape, such as a septate uterus (where a wall of tissue partially divides the cavity), are associated with higher first-trimester loss rates. Adenomyosis, a condition where tissue from the uterine lining grows into the muscular wall, can also contribute to implantation failure, particularly in younger women.
Blood Clotting and Immune Disorders
Antiphospholipid syndrome (APS) is an autoimmune condition that increases the tendency to form blood clots. It’s one of the few treatable medical causes of recurrent early miscarriage. In APS, the immune system produces antibodies that interfere with normal blood flow to the developing placenta, which can starve the embryo of oxygen and nutrients almost as soon as it implants. About half of pregnancy losses linked to these antibodies happen in the first trimester.
APS disrupts a natural anticoagulant that normally coats the surface where the placenta forms. Without that protection, small clots develop in the placental tissue, cutting off the embryo’s blood supply. Inherited clotting disorders, like factor V Leiden, can produce a similar effect. These conditions are particularly relevant for people who experience more than one early loss, because unlike random chromosomal errors, they tend to recur.
How Common This Is
Pregnancy loss affects approximately one-third of all pregnancies, with the vast majority happening in the first trimester. Losses at 4 weeks specifically are so common and so early that most go undetected. Before a pregnancy reaches clinical care, it has already passed through the highest-risk window. The true rate of loss at this stage is difficult to measure precisely because so many chemical pregnancies are never recognized as pregnancies at all.
More than 80% of all miscarriages occur before 12 weeks, and losses cluster heavily in the earliest weeks when chromosomal errors are most likely to end a pregnancy. The further a pregnancy progresses, the lower the risk becomes.
What Happens After a 4-Week Loss
A single chemical pregnancy generally doesn’t require any medical treatment. The bleeding resolves on its own, and your cycle typically returns to normal within four to six weeks. No testing or workup is usually recommended after one early loss, because the overwhelming likelihood is that it was caused by a random chromosomal error that won’t repeat.
If you’ve had two or more miscarriages, blood tests for both partners may be recommended to check for clotting disorders, hormonal issues, or chromosomal rearrangements that could increase the odds of recurrent loss. A single 4-week miscarriage does not meaningfully change your chances of having a successful pregnancy next time. Most people who experience a chemical pregnancy go on to have healthy pregnancies without any intervention.