Fertilization and Implantation
Each human ovary contains 2 million oocytes (eggs) at birth, and approximately 400,000 at the onset of puberty. These are depleted at a rate of approximately 1,000 follicles per month until age 35, when this rate is believed to accelerate.
Assuming regular ovulation during each month between menstruation onset and menopause, and without contraception use, there are approximately 400 to 480 opportunities for pregnancy in a lifetime.
90% of women have a menstrual cycle between 23 and 35 days. This variation almost solely occurs during the first half of the cycle (proliferative/follicular phase), which can last anywhere from 6 to 20 days.
On average, there is only about a 15% to 20% chance of pregnancy each cycle. Fertilization is a complex sequence of molecular events and any disruption to any stage – even minor – can cause the death of the fertilized egg (even before implantation).
The number of potentially fertile days each month may be as high as 12 days, while the number of days of peak cervical mucus (high probability of pregnancy) appears to be about 6 days. However, there may be substantial variability in number of fertile days between cycles for the same woman.
Ovulation is triggered by a surge of luteinizing hormone production; ovulation usually follows within 12 to 24 hours. A woman can still become pregnant if she had sex for up to five days before ovulation, as some sperm may still be alive in the fallopian tubes for this period of time; however, the average is 1 to 3 days.
Usually, only one oocyte is released; if two are released, and fertilized (by different sperm) fraternal twins will result. These children share a womb, but are genetically distinct individuals. Identical twins occurs when one fertilized egg splits into two eggs that are an almost identical genetic match.
Even though it is estimated that about 6 to 12 oocytes have the potential to be released every month, only one oocyte is released during ovulation most of the time.
The chemical/hormonal process that occurs which results in only one egg being released (as opposed to 6 or more) is not completely understood. However, there are several fertility treatments that promote the release of multiple eggs.
Signs of ovulation (cervical position and mucus, BBT) are generally assessed to be more reliable than its symptom, which has been described as a cramping-type of pain (called mittelschmerz, or "middle pain") on the side of the abdomen where ovulation occurs; it can occur before, during, or just after ovulation. It is estimated that up to 40% of women of reproductive age experience this symptom.
Mittelschmerz can occur every month, or just every few cycles, and usually goes away within three to twelve hours. Some women can experience such severe or intense pain that it may be mistaken for appendicitis.
The exact reason for the pain is not known, but tubal or uterine spasm, or increased tension in the ovary is one theory. Another theory includes irritation within the abdomen from the discharge of blood and fluid from the ruptured follicle.
On average, as many as 300 to 350 million sperm (3.5 milliliters of semen) may be ejaculated into the vagina during sex, but only a few hundred make it to the Fallopian tube.
An animal study published in May 2021 found that motile cilia (hair-like projections) and smooth muscle participate in transporting the fertilized egg and sperm through the Fallopian tube. Motile cilia are required for egg pickup, whereas smooth muscle contraction is more important for sperm and embryo transport. (Knowing this mechanism of early embryo transport within the Fallopian tube is critical because disrupted transport is known to lead to female infertility and ectopic pregnancy.)
The oocyte is usually fertilized 12 hours after ovulation and can occur in different parts of the Fallopian tube, but not the uterus (fertilization must occur well before arrival in the uterus).
The most important step in fertilization is the passage of the sperm through the oocyte wall (zona pellucida). Immediate changes then occur in the wall’s properties that stop other sperm from entering. Complete fertilization takes about 24 hours. The resulting zygote has half its chromosomes from the mother, and half from the father.
The sex of the baby is determined at fertilization. Males have X and Y sperm, and each are formed in equal numbers. All oocytes are X. If an X sperm fertilizes the oocyte, the child has female reproductive organs (XX); if a Y sperm fertilizes the oocyte, the fetus will have male reproductive organs (XY).
The widely held belief that X sperm are generally slower than Y sperm has never been confirmed. The belief started in the 1960s – and seemed true in various experiments – until computer assisted sperm analysis indicated there are no differences between the two.
Scientists still do not completely understand all the chemical and molecular events that lead to successful implantation.
Current knowledge indicates that after ovulation and fertilization, the embryo remains in the middle portion of the Fallopian tube while it undergoes a sequence of cell divisions. Cell division occurs through mitosis, where two cells divide into another two cells, and so on. These cell divisions are not dependent on hormones, which is why they can also occur in a lab (in vitro).
About 30 hours after fertilization, the zygote continues to divide into additional cells called blastomeres, becoming a compact ball. The zygote travels toward the uterus, through the final portion of the Fallopian tube, for approximately 10 hours. By this time, the zygote is now a morula (about 16 cells) which is Latin for mulberry, based on its appearance.
Even though the morula continues to divide and obtains more individual cells, the cells just get smaller as they divide, and therefore the morula does not actually get any bigger. A fluid filled cavity then opens in the morula, and it becomes a blastocyst.
During the menstrual cycle, the uterine lining is only receptive to implantation for a short window of time. The most optimal time is known as the window of implantation which is cycle days 20 through 24 (with a 28-day cycle) or 6 to 10 days after ovulation.
Studies have been conducted to determine the most optimal endometrial thickness for implantation, to include the minimum thickness required to support a pregnancy, but no consensus has been reached.
The total time from ovulation to the blastocyst entering the uterus is about 4 days. The blastocyst finds a location at which to implant which is guided by chemical properties of the endometrium. The entire surface of the blastocyst has the potential to attach as it floats around freely. Once the blastocyst attaches, it cannot be dislodged (miscarriage is a different mechanism).
After implantation, the blastocyst separates into two parts: the trophoblast (creates the placenta) and the embryoblast (creates the embryo).
At implantation, the embryo is approximately 0.01 to 0.2 millimeters in length.
It is recommended that women call their HCP as soon as they get a positive home pregnancy test or if they believe they may be pregnant. Although most first appointments will not be for another few weeks, for some women, an HCP may want to see them earlier for possible information/counseling, blood work, pregnancy confirmation, or an immediate change in medication regimen.
Women who are not yet pregnant should consider a pre-conception appointment with their HCP, who can take lab work (i.e. thyroid), answer questions, evaluate nutrition and current medications, conduct an ultrasound prior to pregnancy (possible uterine concerns) and – overall –provide women the best possible head start.
At the first prenatal appointment, HCPs will confirm their due date based the first day of their last menstrual period and/or after ultrasound assessment. Women should read more about Due Date and Gestation to learn what this means and when they can actually expect the baby to arrive.
For women who have just learned they are pregnant, it is estimated that up to 90% of women will experience nausea and vomiting of pregnancy ("morning sickness"), beginning as early as 4 to 6 weeks. The most optimal way to manage these symptoms is to learn about the condition early, apply management techniques early, and get an HCP involved early. Read more to learn why.
Women should consider sharing their experience below of when/how they found out they were pregnant, whether they had any early signs or clues, and how they felt. The experiences page is meant to be a positive place for support and provides women different perspectives of pregnancy they may find valuable.
Short time-lapse video of in-vitro fertilization and initial cell division: